DESCRIPTION

When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking. The "overall options" allow you to stop this
process at an intermediate stage. For example, the -c option says not
to run the linker. Then the output consists of object files output by
the assembler.
Other options are passed on to one stage of processing. Some options
control the preprocessor and others the compiler itself. Yet other
options control the assembler and linker; most of these are not
documented here, since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful
for C programs; when an option is only useful with another language
(usually C++), the explanation says so explicitly. If the description
for a particular option does not mention a source language, you can use
that option with all supported languages.
The gcc program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter
options may not be grouped: -dv is very different from -d-v.
You can mix options and other arguments. For the most part, the order
you use doesn't matter. Order does matter when you use several options
of the same kind; for example, if you specify -L more than once, the
directories are searched in the order specified. Also, the placement
of the -l option is significant.
Many options have long names starting with -f or with -W---for example,
-fmove-loop-invariants, -Wformat and so on. Most of these have both
positive and negative forms; the negative form of -ffoo would be
-fno-foo. This manual documents only one of these two forms, whichever
one is not the default.

OPTIONS

OptionSummary
Here is a summary of all the options, grouped by type. Explanations
are in the following sections.
OverallOptions-c-S-E-ofile-no-canonical-prefixes-pipe-pass-exit-codes-xlanguage-v-###--help[=class[,...]] --target-help--version-wrapper@file-fplugin=file-fplugin-arg-name=arg-fdump-ada-spec[-slim] -fdump-go-spec=fileCLanguageOptions-ansi-std=standard-fgnu89-inline-aux-infofilename-fallow-parameterless-variadic-functions-fno-asm-fno-builtin-fno-builtin-function-fhosted-ffreestanding-fopenmp-fms-extensions-fplan9-extensions-trigraphs-no-integrated-cpp-traditional-traditional-cpp-fallow-single-precision-fcond-mismatch-flax-vector-conversions-fsigned-bitfields-fsigned-char-funsigned-bitfields-funsigned-charC++LanguageOptions-fabi-version=n-fno-access-control-fcheck-new-fconserve-space-fconstexpr-depth=n-ffriend-injection-fno-elide-constructors-fno-enforce-eh-specs-ffor-scope-fno-for-scope-fno-gnu-keywords-fno-implicit-templates-fno-implicit-inline-templates-fno-implement-inlines-fms-extensions-fno-nonansi-builtins-fnothrow-opt-fno-operator-names-fno-optional-diags-fpermissive-fno-pretty-templates-frepo-fno-rtti-fstats-ftemplate-depth=n-fno-threadsafe-statics-fuse-cxa-atexit-fno-weak-nostdinc++-fno-default-inline-fvisibility-inlines-hidden-fvisibility-ms-compat-Wabi-Wconversion-null-Wctor-dtor-privacy-Wdelete-non-virtual-dtor-Wnarrowing-Wnoexcept-Wnon-virtual-dtor-Wreorder-Weffc++-Wstrict-null-sentinel-Wno-non-template-friend-Wold-style-cast-Woverloaded-virtual-Wno-pmf-conversions-Wsign-promoObjective-CandObjective-C++LanguageOptions-fconstant-string-class=class-name-fgnu-runtime-fnext-runtime-fno-nil-receivers-fobjc-abi-version=n-fobjc-call-cxx-cdtors-fobjc-direct-dispatch-fobjc-exceptions-fobjc-gc-fobjc-nilcheck-fobjc-std=objc1-freplace-objc-classes-fzero-link-gen-decls-Wassign-intercept-Wno-protocol-Wselector-Wstrict-selector-match-Wundeclared-selectorLanguageIndependentOptions-fmessage-length=n-fdiagnostics-show-location=[once|every-line]
-fno-diagnostics-show-optionWarningOptions-fsyntax-only-fmax-errors=n-pedantic-pedantic-errors-w-Wextra-Wall-Waddress-Waggregate-return-Warray-bounds-Wno-attributes-Wno-builtin-macro-redefined-Wc++-compat-Wc++11-compat-Wcast-align-Wcast-qual-Wchar-subscripts-Wclobbered-Wcomment-Wconversion-Wcoverage-mismatch-Wno-cpp-Wno-deprecated-Wno-deprecated-declarations-Wdisabled-optimization-Wno-div-by-zero-Wdouble-promotion-Wempty-body-Wenum-compare-Wno-endif-labels-Werror-Werror=*-Wfatal-errors-Wfloat-equal-Wformat-Wformat=2-Wno-format-contains-nul-Wno-format-extra-args-Wformat-nonliteral-Wformat-security-Wformat-y2k-Wframe-larger-than=len-Wno-free-nonheap-object-Wjump-misses-init-Wignored-qualifiers-Wimplicit-Wimplicit-function-declaration-Wimplicit-int-Winit-self-Winline-Wmaybe-uninitialized-Wno-int-to-pointer-cast-Wno-invalid-offsetof-Winvalid-pch-Wlarger-than=len-Wunsafe-loop-optimizations-Wlogical-op-Wlong-long-Wmain-Wmaybe-uninitialized-Wmissing-braces-Wmissing-field-initializers-Wmissing-format-attribute-Wmissing-include-dirs-Wno-mudflap-Wno-multichar-Wnonnull-Wno-overflow-Woverlength-strings-Wpacked-Wpacked-bitfield-compat-Wpadded-Wparentheses-Wpedantic-ms-format-Wno-pedantic-ms-format-Wpointer-arith-Wno-pointer-to-int-cast-Wredundant-decls-Wreturn-type-Wsequence-point-Wshadow-Wsign-compare-Wsign-conversion-Wstack-protector-Wstack-usage=len-Wstrict-aliasing-Wstrict-aliasing=n-Wstrict-overflow-Wstrict-overflow=n-Wsuggest-attribute=[pure|const|noreturn] -Wswitch-Wswitch-default-Wswitch-enum-Wsync-nand-Wsystem-headers-Wtrampolines-Wtrigraphs-Wtype-limits-Wundef-Wuninitialized-Wunknown-pragmas-Wno-pragmas-Wunsuffixed-float-constants-Wunused-Wunused-function-Wunused-label-Wunused-local-typedefs-Wunused-parameter-Wno-unused-result-Wunused-value-Wunused-variable-Wunused-but-set-parameter-Wunused-but-set-variable-Wvariadic-macros-Wvector-operation-performance-Wvla-Wvolatile-register-var-Wwrite-strings-Wzero-as-null-pointer-constantCandObjective-C-onlyWarningOptions-Wbad-function-cast-Wmissing-declarations-Wmissing-parameter-type-Wmissing-prototypes-Wnested-externs-Wold-style-declaration-Wold-style-definition-Wstrict-prototypes-Wtraditional-Wtraditional-conversion-Wdeclaration-after-statement-Wpointer-signDebuggingOptions-dletters-dumpspecs-dumpmachine-dumpversion-fdbg-cnt-list-fdbg-cnt=counter-value-list-fdisable-ipa-pass_name-fdisable-rtl-pass_name-fdisable-rtl-pass-name=range-list-fdisable-tree-pass_name-fdisable-tree-pass-name=range-list-fdump-noaddr-fdump-unnumbered-fdump-unnumbered-links-fdump-translation-unit[-n] -fdump-class-hierarchy[-n]
-fdump-ipa-all-fdump-ipa-cgraph-fdump-ipa-inline-fdump-passes-fdump-statistics-fdump-tree-all-fdump-tree-original[-n]
-fdump-tree-optimized[-n] -fdump-tree-cfg-fdump-tree-vcg-fdump-tree-alias-fdump-tree-ch-fdump-tree-ssa[-n]
-fdump-tree-pre[-n] -fdump-tree-ccp[-n] -fdump-tree-dce[-n]
-fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n]
-fdump-tree-dom[-n] -fdump-tree-dse[-n] -fdump-tree-phiprop[-n]
-fdump-tree-phiopt[-n] -fdump-tree-forwprop[-n]
-fdump-tree-copyrename[-n] -fdump-tree-nrv-fdump-tree-vect-fdump-tree-sink-fdump-tree-sra[-n] -fdump-tree-forwprop[-n]
-fdump-tree-fre[-n] -fdump-tree-vrp[-n] -ftree-vectorizer-verbose=n-fdump-tree-storeccp[-n] -fdump-final-insns=file-fcompare-debug[=opts] -fcompare-debug-second-feliminate-dwarf2-dups-feliminate-unused-debug-types-feliminate-unused-debug-symbols-femit-class-debug-always-fenable-kind-pass-fenable-kind-pass=range-list-fdebug-types-section-fmem-report-fpre-ipa-mem-report-fpost-ipa-mem-report-fprofile-arcs-frandom-seed=string-fsched-verbose=n-fsel-sched-verbose-fsel-sched-dump-cfg-fsel-sched-pipelining-verbose-fstack-usage-ftest-coverage-ftime-report-fvar-tracking-fvar-tracking-assignments-fvar-tracking-assignments-toggle-g-glevel-gtoggle-gcoff-gdwarf-version-ggdb-grecord-gcc-switches-gno-record-gcc-switches-gstabs-gstabs+-gstrict-dwarf-gno-strict-dwarf-gvms-gxcoff-gxcoff+-fno-merge-debug-strings-fno-dwarf2-cfi-asm-fdebug-prefix-map=old=new-femit-struct-debug-baseonly-femit-struct-debug-reduced-femit-struct-debug-detailed[=spec-list] -p-pg-print-file-name=library-print-libgcc-file-name-print-multi-directory-print-multi-lib-print-multi-os-directory-print-prog-name=program-print-search-dirs-Q-print-sysroot-print-sysroot-headers-suffix-save-temps-save-temps=cwd-save-temps=obj-time[=file]
OptimizationOptions-falign-functions[=n]-falign-jumps[=n]-falign-labels[=n]-falign-loops[=n]-fassociative-math-fauto-inc-dec-fbranch-probabilities-fbranch-target-load-optimize-fbranch-target-load-optimize2-fbtr-bb-exclusive-fcaller-saves-fcheck-data-deps-fcombine-stack-adjustments-fconserve-stack-fcompare-elim-fcprop-registers-fcrossjumping-fcse-follow-jumps-fcse-skip-blocks-fcx-fortran-rules-fcx-limited-range-fdata-sections-fdce-fdelayed-branch-fdelete-null-pointer-checks-fdevirtualize-fdse-fearly-inlining-fipa-sra-fexpensive-optimizations-ffat-lto-objects-ffast-math-ffinite-math-only-ffloat-store-fexcess-precision=style-fforward-propagate-ffp-contract=style-ffunction-sections-fgcse-fgcse-after-reload-fgcse-las-fgcse-lm-fgraphite-identity-fgcse-sm-fif-conversion-fif-conversion2-findirect-inlining-finline-functions-finline-functions-called-once-finline-limit=n-finline-small-functions-fipa-cp-fipa-cp-clone-fipa-matrix-reorg-fipa-pta-fipa-profile-fipa-pure-const-fipa-reference-fira-algorithm=algorithm-fira-region=region-fira-loop-pressure-fno-ira-share-save-slots-fno-ira-share-spill-slots-fira-verbose=n-fivopts-fkeep-inline-functions-fkeep-static-consts-floop-block-floop-flatten-floop-interchange-floop-strip-mine-floop-parallelize-all-flto-flto-compression-level-flto-partition=alg-flto-report-fmerge-all-constants-fmerge-constants-fmodulo-sched-fmodulo-sched-allow-regmoves-fmove-loop-invariantsfmudflap-fmudflapir-fmudflapth-fno-branch-count-reg-fno-default-inline-fno-defer-pop-fno-function-cse-fno-guess-branch-probability-fno-inline-fno-math-errno-fno-peephole-fno-peephole2-fno-sched-interblock-fno-sched-spec-fno-signed-zeros-fno-toplevel-reorder-fno-trapping-math-fno-zero-initialized-in-bss-fomit-frame-pointer-foptimize-register-move-foptimize-sibling-calls-fpartial-inlining-fpeel-loops-fpredictive-commoning-fprefetch-loop-arrays-fprofile-correction-fprofile-dir=path-fprofile-generate-fprofile-generate=path-fprofile-use-fprofile-use=path-fprofile-values-freciprocal-math-free-fregmove-frename-registers-freorder-blocks-freorder-blocks-and-partition-freorder-functions-frerun-cse-after-loop-freschedule-modulo-scheduled-loops-frounding-math-fsched2-use-superblocks-fsched-pressure-fsched-spec-load-fsched-spec-load-dangerous-fsched-stalled-insns-dep[=n]-fsched-stalled-insns[=n]-fsched-group-heuristic-fsched-critical-path-heuristic-fsched-spec-insn-heuristic-fsched-rank-heuristic-fsched-last-insn-heuristic-fsched-dep-count-heuristic-fschedule-insns-fschedule-insns2-fsection-anchors-fselective-scheduling-fselective-scheduling2-fsel-sched-pipelining-fsel-sched-pipelining-outer-loops-fshrink-wrap-fsignaling-nans-fsingle-precision-constant-fsplit-ivs-in-unroller-fsplit-wide-types-fstack-protector-fstack-protector-all-fstrict-aliasing-fstrict-overflow-fthread-jumps-ftracer-ftree-bit-ccp-ftree-builtin-call-dce-ftree-ccp-ftree-ch-ftree-copy-prop-ftree-copyrename-ftree-dce-ftree-dominator-opts-ftree-dse-ftree-forwprop-ftree-fre-ftree-loop-if-convert-ftree-loop-if-convert-stores-ftree-loop-im-ftree-phiprop-ftree-loop-distribution-ftree-loop-distribute-patterns-ftree-loop-ivcanon-ftree-loop-linear-ftree-loop-optimize-ftree-parallelize-loops=n-ftree-pre-ftree-pta-ftree-reassoc-ftree-sink-ftree-sra-ftree-switch-conversion-ftree-tail-merge-ftree-ter-ftree-vect-loop-version-ftree-vectorize-ftree-vrp-funit-at-a-time-funroll-all-loops-funroll-loops-funsafe-loop-optimizations-funsafe-math-optimizations-funswitch-loops-fvariable-expansion-in-unroller-fvect-cost-model-fvpt-fweb-fwhole-program-fwpa-fuse-linker-plugin--paramname=value-O-O0-O1-O2-O3-Os-OfastPreprocessorOptions-Aquestion=answer-A-question[=answer] -C-dD-dI-dM-dN-Dmacro[=defn] -E-H-idirafterdir-includefile-imacrosfile-iprefixfile-iwithprefixdir-iwithprefixbeforedir-isystemdir-imultilibdir-isysrootdir-M-MM-MF-MG-MP-MQ-MT-nostdinc-P-fdebug-cpp-ftrack-macro-expansion-fworking-directory-remap-trigraphs-undef-Umacro-Wp,option-XpreprocessoroptionAssemblerOption-Wa,option-XassembleroptionLinkerOptionsobject-file-name-llibrary-nostartfiles-nodefaultlibs-nostdlib-pie-rdynamic-s-static-static-libgcc-static-libstdc++-shared-shared-libgcc-symbolic-Tscript-Wl,option-Xlinkeroption-usymbolDirectoryOptions-Bprefix-Idir-iplugindir=dir-iquotedir-Ldir-specs=file-I---sysroot=dirMachineDependentOptionsAdaptevaEpiphanyOptions-mhalf-reg-file-mprefer-short-insn-regs-mbranch-cost=num-mcmove-mnops=num-msoft-cmpsf-msplit-lohi-mpost-inc-mpost-modify-mstack-offset=num-mround-nearest-mlong-calls-mshort-calls-msmall16-mfp-mode=mode-mvect-double-max-vect-align=num-msplit-vecmove-early-m1reg-regARMOptions-mapcs-frame-mno-apcs-frame-mabi=name-mapcs-stack-check-mno-apcs-stack-check-mapcs-float-mno-apcs-float-mapcs-reentrant-mno-apcs-reentrant-msched-prolog-mno-sched-prolog-mlittle-endian-mbig-endian-mwords-little-endian-mfloat-abi=name-mfpe-mfp16-format=name-mthumb-interwork-mno-thumb-interwork-mcpu=name-march=name-mfpu=name-mstructure-size-boundary=n-mabort-on-noreturn-mlong-calls-mno-long-calls-msingle-pic-base-mno-single-pic-base-mpic-register=reg-mnop-fun-dllimport-mcirrus-fix-invalid-insns-mno-cirrus-fix-invalid-insns-mpoke-function-name-mthumb-marm-mtpcs-frame-mtpcs-leaf-frame-mcaller-super-interworking-mcallee-super-interworking-mtp=name-mtls-dialect=dialect-mword-relocations-mfix-cortex-m3-ldrd-munaligned-accessAVROptions-mmcu=mcu-maccumulate-args-mbranch-cost=cost-mcall-prologues-mint8-mno-interrupts-mrelax-mshort-calls-mstrict-X-mtiny-stackBlackfinOptions-mcpu=cpu[-sirevision] -msim-momit-leaf-frame-pointer-mno-omit-leaf-frame-pointer-mspecld-anomaly-mno-specld-anomaly-mcsync-anomaly-mno-csync-anomaly-mlow-64k-mno-low64k-mstack-check-l1-mid-shared-library-mno-id-shared-library-mshared-library-id=n-mleaf-id-shared-library-mno-leaf-id-shared-library-msep-data-mno-sep-data-mlong-calls-mno-long-calls-mfast-fp-minline-plt-mmulticore-mcorea-mcoreb-msdram-micplbC6XOptions-mbig-endian-mlittle-endian-march=cpu-msim-msdata=sdata-typeCRISOptions-mcpu=cpu-march=cpu-mtune=cpu-mmax-stack-frame=n-melinux-stacksize=n-metrax4-metrax100-mpdebug-mcc-init-mno-side-effects-mstack-align-mdata-align-mconst-align-m32-bit-m16-bit-m8-bit-mno-prologue-epilogue-mno-gotplt-melf-maout-melinux-mlinux-sim-sim2-mmul-bug-workaround-mno-mul-bug-workaroundCR16Options-mmac-mcr16cplus-mcr16c-msim-mint32-mbit-ops-mdata-model=modelDarwinOptions-all_load-allowable_client-arch-arch_errors_fatal-arch_only-bind_at_load-bundle-bundle_loader-client_name-compatibility_version-current_version-dead_strip-dependency-file-dylib_file-dylinker_install_name-dynamic-dynamiclib-exported_symbols_list-filelist-flat_namespace-force_cpusubtype_ALL-force_flat_namespace-headerpad_max_install_names-iframework-image_base-init-install_name-keep_private_externs-multi_module-multiply_defined-multiply_defined_unused-noall_load-no_dead_strip_inits_and_terms-nofixprebinding-nomultidefs-noprebind-noseglinkedit-pagezero_size-prebind-prebind_all_twolevel_modules-private_bundle-read_only_relocs-sectalign-sectobjectsymbols-whyload-seg1addr-sectcreate-sectobjectsymbols-sectorder-segaddr-segs_read_only_addr-segs_read_write_addr-seg_addr_table-seg_addr_table_filename-seglinkedit-segprot-segs_read_only_addr-segs_read_write_addr-single_module-static-sub_library-sub_umbrella-twolevel_namespace-umbrella-undefined-unexported_symbols_list-weak_reference_mismatches-whatsloaded-F-gused-gfull-mmacosx-version-min=version-mkernel-mone-byte-boolDECAlphaOptions-mno-fp-regs-msoft-float-malpha-as-mgas-mieee-mieee-with-inexact-mieee-conformant-mfp-trap-mode=mode-mfp-rounding-mode=mode-mtrap-precision=mode-mbuild-constants-mcpu=cpu-type-mtune=cpu-type-mbwx-mmax-mfix-mcix-mfloat-vax-mfloat-ieee-mexplicit-relocs-msmall-data-mlarge-data-msmall-text-mlarge-text-mmemory-latency=timeDECAlpha/VMSOptions-mvms-return-codes-mdebug-main=prefix-mmalloc64FR30Options-msmall-model-mno-lsimFRVOptions-mgpr-32-mgpr-64-mfpr-32-mfpr-64-mhard-float-msoft-float-malloc-cc-mfixed-cc-mdword-mno-dword-mdouble-mno-double-mmedia-mno-media-mmuladd-mno-muladd-mfdpic-minline-plt-mgprel-ro-multilib-library-pic-mlinked-fp-mlong-calls-malign-labels-mlibrary-pic-macc-4-macc-8-mpack-mno-pack-mno-eflags-mcond-move-mno-cond-move-moptimize-membar-mno-optimize-membar-mscc-mno-scc-mcond-exec-mno-cond-exec-mvliw-branch-mno-vliw-branch-mmulti-cond-exec-mno-multi-cond-exec-mnested-cond-exec-mno-nested-cond-exec-mtomcat-stats-mTLS-mtls-mcpu=cpuGNU/LinuxOptions-mglibc-muclibc-mbionic-mandroid-tno-android-cc-tno-android-ldH8/300Options-mrelax-mh-ms-mn-mint32-malign-300HPPAOptions-march=architecture-type-mbig-switch-mdisable-fpregs-mdisable-indexing-mfast-indirect-calls-mgas-mgnu-ld-mhp-ld-mfixed-range=register-range-mjump-in-delay-mlinker-opt-mlong-calls-mlong-load-store-mno-big-switch-mno-disable-fpregs-mno-disable-indexing-mno-fast-indirect-calls-mno-gas-mno-jump-in-delay-mno-long-load-store-mno-portable-runtime-mno-soft-float-mno-space-regs-msoft-float-mpa-risc-1-0-mpa-risc-1-1-mpa-risc-2-0-mportable-runtime-mschedule=cpu-type-mspace-regs-msio-mwsio-munix=unix-std-nolibdld-static-threadsi386andx86-64Options-mtune=cpu-type-march=cpu-type-mfpmath=unit-masm=dialect-mno-fancy-math-387-mno-fp-ret-in-387-msoft-float-mno-wide-multiply-mrtd-malign-double-mpreferred-stack-boundary=num-mincoming-stack-boundary=num-mcld-mcx16-msahf-mmovbe-mcrc32-mrecip-mrecip=opt-mvzeroupper-mprefer-avx128-mmmx-msse-msse2-msse3-mssse3-msse4.1-msse4.2-msse4-mavx-mavx2-maes-mpclmul-mfsgsbase-mrdrnd-mf16c-mfma-msse4a-m3dnow-mpopcnt-mabm-mbmi-mtbm-mfma4-mxop-mlzcnt-mbmi2-mlwp-mthreads-mno-align-stringops-minline-all-stringops-minline-stringops-dynamically-mstringop-strategy=alg-mpush-args-maccumulate-outgoing-args-m128bit-long-double-m96bit-long-double-mregparm=num-msseregparm-mveclibabi=type-mvect8-ret-in-mem-mpc32-mpc64-mpc80-mstackrealign-momit-leaf-frame-pointer-mno-red-zone-mno-tls-direct-seg-refs-mcmodel=code-model-mabi=name-m32-m64-mx32-mlarge-data-threshold=num-msse2avx-mfentry-m8bit-idiv-mavx256-split-unaligned-load-mavx256-split-unaligned-storei386andx86-64WindowsOptions-mconsole-mcygwin-mno-cygwin-mdll-mnop-fun-dllimport-mthread-municode-mwin32-mwindows-fno-set-stack-executableIA-64Options-mbig-endian-mlittle-endian-mgnu-as-mgnu-ld-mno-pic-mvolatile-asm-stop-mregister-names-msdata-mno-sdata-mconstant-gp-mauto-pic-mfused-madd-minline-float-divide-min-latency-minline-float-divide-max-throughput-mno-inline-float-divide-minline-int-divide-min-latency-minline-int-divide-max-throughput-mno-inline-int-divide-minline-sqrt-min-latency-minline-sqrt-max-throughput-mno-inline-sqrt-mdwarf2-asm-mearly-stop-bits-mfixed-range=register-range-mtls-size=tls-size-mtune=cpu-type-milp32-mlp64-msched-br-data-spec-msched-ar-data-spec-msched-control-spec-msched-br-in-data-spec-msched-ar-in-data-spec-msched-in-control-spec-msched-spec-ldc-msched-spec-control-ldc-msched-prefer-non-data-spec-insns-msched-prefer-non-control-spec-insns-msched-stop-bits-after-every-cycle-msched-count-spec-in-critical-path-msel-sched-dont-check-control-spec-msched-fp-mem-deps-zero-cost-msched-max-memory-insns-hard-limit-msched-max-memory-insns=max-insnsIA-64/VMSOptions-mvms-return-codes-mdebug-main=prefix-mmalloc64LM32Options-mbarrel-shift-enabled-mdivide-enabled-mmultiply-enabled-msign-extend-enabled-muser-enabledM32R/DOptions-m32r2-m32rx-m32r-mdebug-malign-loops-mno-align-loops-missue-rate=number-mbranch-cost=number-mmodel=code-size-model-type-msdata=sdata-type-mno-flush-func-mflush-func=name-mno-flush-trap-mflush-trap=number-GnumM32COptions-mcpu=cpu-msim-memregs=numberM680x0Options-march=arch-mcpu=cpu-mtune=tune-m68000-m68020-m68020-40-m68020-60-m68030-m68040-m68060-mcpu32-m5200-m5206e-m528x-m5307-m5407-mcfv4e-mbitfield-mno-bitfield-mc68000-mc68020-mnobitfield-mrtd-mno-rtd-mdiv-mno-div-mshort-mno-short-mhard-float-m68881-msoft-float-mpcrel-malign-int-mstrict-align-msep-data-mno-sep-data-mshared-library-id=n-mid-shared-library-mno-id-shared-library-mxgot-mno-xgotMCoreOptions-mhardlit-mno-hardlit-mdiv-mno-div-mrelax-immediates-mno-relax-immediates-mwide-bitfields-mno-wide-bitfields-m4byte-functions-mno-4byte-functions-mcallgraph-data-mno-callgraph-data-mslow-bytes-mno-slow-bytes-mno-lsim-mlittle-endian-mbig-endian-m210-m340-mstack-incrementMePOptions-mabsdiff-mall-opts-maverage-mbased=n-mbitops-mc=n-mclip-mconfig=name-mcop-mcop32-mcop64-mivc2-mdc-mdiv-meb-mel-mio-volatile-ml-mleadz-mm-mminmax-mmult-mno-opts-mrepeat-ms-msatur-msdram-msim-msimnovec-mtf-mtiny=nMicroBlazeOptions-msoft-float-mhard-float-msmall-divides-mcpu=cpu-mmemcpy-mxl-soft-mul-mxl-soft-div-mxl-barrel-shift-mxl-pattern-compare-mxl-stack-check-mxl-gp-opt-mno-clearbss-mxl-multiply-high-mxl-float-convert-mxl-float-sqrt-mxl-mode-app-modelMIPSOptions-EL-EB-march=arch-mtune=arch-mips1-mips2-mips3-mips4-mips32-mips32r2-mips64-mips64r2-mips16-mno-mips16-mflip-mips16-minterlink-mips16-mno-interlink-mips16-mabi=abi-mabicalls-mno-abicalls-mshared-mno-shared-mplt-mno-plt-mxgot-mno-xgot-mgp32-mgp64-mfp32-mfp64-mhard-float-msoft-float-msingle-float-mdouble-float-mdsp-mno-dsp-mdspr2-mno-dspr2-mfpu=fpu-type-msmartmips-mno-smartmips-mpaired-single-mno-paired-single-mdmx-mno-mdmx-mips3d-mno-mips3d-mmt-mno-mt-mllsc-mno-llsc-mlong64-mlong32-msym32-mno-sym32-Gnum-mlocal-sdata-mno-local-sdata-mextern-sdata-mno-extern-sdata-mgpopt-mno-gopt-membedded-data-mno-embedded-data-muninit-const-in-rodata-mno-uninit-const-in-rodata-mcode-readable=setting-msplit-addresses-mno-split-addresses-mexplicit-relocs-mno-explicit-relocs-mcheck-zero-division-mno-check-zero-division-mdivide-traps-mdivide-breaks-mmemcpy-mno-memcpy-mlong-calls-mno-long-calls-mmad-mno-mad-mfused-madd-mno-fused-madd-nocpp-mfix-24k-mno-fix-24k-mfix-r4000-mno-fix-r4000-mfix-r4400-mno-fix-r4400-mfix-r10000-mno-fix-r10000-mfix-vr4120-mno-fix-vr4120-mfix-vr4130-mno-fix-vr4130-mfix-sb1-mno-fix-sb1-mflush-func=func-mno-flush-func-mbranch-cost=num-mbranch-likely-mno-branch-likely-mfp-exceptions-mno-fp-exceptions-mvr4130-align-mno-vr4130-align-msynci-mno-synci-mrelax-pic-calls-mno-relax-pic-calls-mmcount-ra-addressMMIXOptions-mlibfuncs-mno-libfuncs-mepsilon-mno-epsilon-mabi=gnu-mabi=mmixware-mzero-extend-mknuthdiv-mtoplevel-symbols-melf-mbranch-predict-mno-branch-predict-mbase-addresses-mno-base-addresses-msingle-exit-mno-single-exitMN10300Options-mmult-bug-mno-mult-bug-mno-am33-mam33-mam33-2-mam34-mtune=cpu-type-mreturn-pointer-on-d0-mno-crt0-mrelax-mliw-msetlbPDP-11Options-mfpu-msoft-float-mac0-mno-ac0-m40-m45-m10-mbcopy-mbcopy-builtin-mint32-mno-int16-mint16-mno-int32-mfloat32-mno-float64-mfloat64-mno-float32-mabshi-mno-abshi-mbranch-expensive-mbranch-cheap-munix-asm-mdec-asmpicoChipOptions-mae=ae_type-mvliw-lookahead=N-msymbol-as-address-mno-inefficient-warningsPowerPCOptions See RS/6000 and PowerPC Options.
RL78Options-msim-mmul=none-mmul=g13-mmul=rl78RS/6000andPowerPCOptions-mcpu=cpu-type-mtune=cpu-type-mcmodel=code-model-mpower-mno-power-mpower2-mno-power2-mpowerpc-mpowerpc64-mno-powerpc-maltivec-mno-altivec-mpowerpc-gpopt-mno-powerpc-gpopt-mpowerpc-gfxopt-mno-powerpc-gfxopt-mmfcrf-mno-mfcrf-mpopcntb-mno-popcntb-mpopcntd-mno-popcntd-mfprnd-mno-fprnd-mcmpb-mno-cmpb-mmfpgpr-mno-mfpgpr-mhard-dfp-mno-hard-dfp-mnew-mnemonics-mold-mnemonics-mfull-toc-mminimal-toc-mno-fp-in-toc-mno-sum-in-toc-m64-m32-mxl-compat-mno-xl-compat-mpe-malign-power-malign-natural-msoft-float-mhard-float-mmultiple-mno-multiple-msingle-float-mdouble-float-msimple-fpu-mstring-mno-string-mupdate-mno-update-mavoid-indexed-addresses-mno-avoid-indexed-addresses-mfused-madd-mno-fused-madd-mbit-align-mno-bit-align-mstrict-align-mno-strict-align-mrelocatable-mno-relocatable-mrelocatable-lib-mno-relocatable-lib-mtoc-mno-toc-mlittle-mlittle-endian-mbig-mbig-endian-mdynamic-no-pic-maltivec-mswdiv-msingle-pic-base-mprioritize-restricted-insns=priority-msched-costly-dep=dependence_type-minsert-sched-nops=scheme-mcall-sysv-mcall-netbsd-maix-struct-return-msvr4-struct-return-mabi=abi-type-msecure-plt-mbss-plt-mblock-move-inline-limit=num-misel-mno-isel-misel=yes-misel=no-mspe-mno-spe-mspe=yes-mspe=no-mpaired-mgen-cell-microcode-mwarn-cell-microcode-mvrsave-mno-vrsave-mmulhw-mno-mulhw-mdlmzb-mno-dlmzb-mfloat-gprs=yes-mfloat-gprs=no-mfloat-gprs=single-mfloat-gprs=double-mprototype-mno-prototype-msim-mmvme-mads-myellowknife-memb-msdata-msdata=opt-mvxworks-Gnum-pthread-mrecip-mrecip=opt-mno-recip-mrecip-precision-mno-recip-precision-mveclibabi=type-mfriz-mno-friz-mpointers-to-nested-functions-mno-pointers-to-nested-functions-msave-toc-indirect-mno-save-toc-indirectRXOptions-m64bit-doubles-m32bit-doubles-fpu-nofpu-mcpu=-mbig-endian-data-mlittle-endian-data-msmall-data-msim-mno-sim-mas100-syntax-mno-as100-syntax-mrelax-mmax-constant-size=-mint-register=-mpid-msave-acc-in-interruptsS/390andzSeriesOptions-mtune=cpu-type-march=cpu-type-mhard-float-msoft-float-mhard-dfp-mno-hard-dfp-mlong-double-64-mlong-double-128-mbackchain-mno-backchain-mpacked-stack-mno-packed-stack-msmall-exec-mno-small-exec-mmvcle-mno-mvcle-m64-m31-mdebug-mno-debug-mesa-mzarch-mtpf-trace-mno-tpf-trace-mfused-madd-mno-fused-madd-mwarn-framesize-mwarn-dynamicstack-mstack-size-mstack-guardScoreOptions-meb-mel-mnhwloop-muls-mmac-mscore5-mscore5u-mscore7-mscore7dSHOptions-m1-m2-m2e-m2a-nofpu-m2a-single-only-m2a-single-m2a-m3-m3e-m4-nofpu-m4-single-only-m4-single-m4-m4a-nofpu-m4a-single-only-m4a-single-m4a-m4al-m5-64media-m5-64media-nofpu-m5-32media-m5-32media-nofpu-m5-compact-m5-compact-nofpu-mb-ml-mdalign-mrelax-mbigtable-mfmovd-mhitachi-mrenesas-mno-renesas-mnomacsave-mieee-mno-ieee-mbitops-misize-minline-ic_invalidate-mpadstruct-mspace-mprefergot-musermode-multcost=number-mdiv=strategy-mdivsi3_libfunc=name-mfixed-range=register-range-madjust-unroll-mindexed-addressing-mgettrcost=number-mpt-fixed-maccumulate-outgoing-args-minvalid-symbols-msoft-atomic-mbranch-cost=num-mcbranchdi-mcmpeqdi-mfused-madd-mpretend-cmoveSolaris2Options-mimpure-text-mno-impure-text-pthreads-pthreadSPARCOptions-mcpu=cpu-type-mtune=cpu-type-mcmodel=code-model-mmemory-model=mem-model-m32-m64-mapp-regs-mno-app-regs-mfaster-structs-mno-faster-structs-mflat-mno-flat-mfpu-mno-fpu-mhard-float-msoft-float-mhard-quad-float-msoft-quad-float-mstack-bias-mno-stack-bias-munaligned-doubles-mno-unaligned-doubles-mv8plus-mno-v8plus-mvis-mno-vis-mvis2-mno-vis2-mvis3-mno-vis3-mfmaf-mno-fmaf-mpopc-mno-popc-mfix-at697fSPUOptions-mwarn-reloc-merror-reloc-msafe-dma-munsafe-dma-mbranch-hints-msmall-mem-mlarge-mem-mstdmain-mfixed-range=register-range-mea32-mea64-maddress-space-conversion-mno-address-space-conversion-mcache-size=cache-size-matomic-updates-mno-atomic-updatesSystemVOptions-Qy-Qn-YP,paths-Ym,dirTILE-GxOptions-mcpu=cpu-m32-m64TILEProOptions-mcpu=cpu-m32V850Options-mlong-calls-mno-long-calls-mep-mno-ep-mprolog-function-mno-prolog-function-mspace-mtda=n-msda=n-mzda=n-mapp-regs-mno-app-regs-mdisable-callt-mno-disable-callt-mv850e2v3-mv850e2-mv850e1-mv850es-mv850e-mv850-mbig-switchVAXOptions-mg-mgnu-munixVxWorksOptions-mrtp-non-static-Bstatic-Bdynamic-Xbind-lazy-Xbind-nowx86-64Options See i386 and x86-64 Options.
Xstormy16Options-msimXtensaOptions-mconst16-mno-const16-mfused-madd-mno-fused-madd-mforce-no-pic-mserialize-volatile-mno-serialize-volatile-mtext-section-literals-mno-text-section-literals-mtarget-align-mno-target-align-mlongcalls-mno-longcallszSeriesOptions See S/390 and zSeries Options.
CodeGenerationOptions-fcall-saved-reg-fcall-used-reg-ffixed-reg-fexceptions-fnon-call-exceptions-funwind-tables-fasynchronous-unwind-tables-finhibit-size-directive-finstrument-functions-finstrument-functions-exclude-function-list=sym,sym,...-finstrument-functions-exclude-file-list=file,file,...-fno-common-fno-ident-fpcc-struct-return-fpic-fPIC-fpie-fPIE-fno-jump-tables-frecord-gcc-switches-freg-struct-return-fshort-enums-fshort-double-fshort-wchar-fverbose-asm-fpack-struct[=n]-fstack-check-fstack-limit-register=reg-fstack-limit-symbol=sym-fno-stack-limit-fsplit-stack-fleading-underscore-ftls-model=model-ftrapv-fwrapv-fbounds-check-fvisibility-fstrict-volatile-bitfieldsOptionsControllingtheKindofOutput
Compilation can involve up to four stages: preprocessing, compilation
proper, assembly and linking, always in that order. GCC is capable of
preprocessing and compiling several files either into several assembler
input files, or into one assembler input file; then each assembler
input file produces an object file, and linking combines all the object
files (those newly compiled, and those specified as input) into an
executable file.
For any given input file, the file name suffix determines what kind of
compilation is done:
file.c
C source code that must be preprocessed.
file.i
C source code that should not be preprocessed.
file.ii
C++ source code that should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the libobjc
library to make an Objective-C program work.
file.mi
Objective-C source code that should not be preprocessed.
file.mmfile.M
Objective-C++ source code. Note that you must link with the
libobjc library to make an Objective-C++ program work. Note that
.M refers to a literal capital M.
file.mii
Objective-C++ source code that should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into
a precompiled header (default), or C, C++ header file to be turned
into an Ada spec (via the -fdump-ada-spec switch).
file.ccfile.cpfile.cxxfile.cppfile.CPPfile.c++file.C
C++ source code that must be preprocessed. Note that in .cxx, the
last two letters must both be literally x. Likewise, .C refers to
a literal capital C.
file.mmfile.M
Objective-C++ source code that must be preprocessed.
file.mii
Objective-C++ source code that should not be preprocessed.
file.hhfile.Hfile.hpfile.hxxfile.hppfile.HPPfile.h++file.tcc
C++ header file to be turned into a precompiled header or Ada spec.
file.ffile.forfile.ftn
Fixed form Fortran source code that should not be preprocessed.
file.Ffile.FORfile.fppfile.FPPfile.FTN
Fixed form Fortran source code that must be preprocessed (with the
traditional preprocessor).
file.f90file.f95file.f03file.f08
Free form Fortran source code that should not be preprocessed.
file.F90file.F95file.F03file.F08
Free form Fortran source code that must be preprocessed (with the
traditional preprocessor).
file.go
Go source code.
file.ads
Ada source code file that contains a library unit declaration (a
declaration of a package, subprogram, or generic, or a generic
instantiation), or a library unit renaming declaration (a package,
generic, or subprogram renaming declaration). Such files are also
called specs.
file.adb
Ada source code file containing a library unit body (a subprogram
or package body). Such files are also called bodies.
file.s
Assembler code.
file.Sfile.sx
Assembler code that must be preprocessed.
other
An object file to be fed straight into linking. Any file name with
no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
-xlanguage
Specify explicitly the language for the following input files
(rather than letting the compiler choose a default based on the
file name suffix). This option applies to all following input
files until the next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input f95 f95-cpp-input
go
java
-xnone
Turn off any specification of a language, so that subsequent files
are handled according to their file name suffixes (as they are if
-x has not been used at all).
-pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase
of the compiler returns a non-success return code. If you specify
-pass-exit-codes, the gcc program will instead return with
numerically highest error produced by any phase that returned an
error indication. The C, C++, and Fortran frontends return 4, if
an internal compiler error is encountered.
If you only want some of the stages of compilation, you can use -x (or
filename suffixes) to tell gcc where to start, and one of the options
-c, -S, or -E to say where gcc is to stop. Note that some combinations
(for example, -xcpp-output-E) instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking
stage simply is not done. The ultimate output is in the form of an
object file for each source file.
By default, the object file name for a source file is made by
replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly,
are ignored.
-S Stop after the stage of compilation proper; do not assemble. The
output is in the form of an assembler code file for each non-
assembler input file specified.
By default, the assembler file name for a source file is made by
replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper.
The output is in the form of preprocessed source code, which is
sent to the standard output.
Input files that don't require preprocessing are ignored.
-ofile
Place output in file file. This applies regardless to whatever
sort of output is being produced, whether it be an executable file,
an object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in
a.out, the object file for source.suffix in source.o, its assembler
file in source.s, a precompiled header file in source.suffix.gch,
and all preprocessed C source on standard output.
-v Print (on standard error output) the commands executed to run the
stages of compilation. Also print the version number of the
compiler driver program and of the preprocessor and the compiler
proper.
-###
Like -v except the commands are not executed and arguments are
quoted unless they contain only alphanumeric characters or "./-_".
This is useful for shell scripts to capture the driver-generated
command lines.
-pipe
Use pipes rather than temporary files for communication between the
various stages of compilation. This fails to work on some systems
where the assembler is unable to read from a pipe; but the GNU
assembler has no trouble.
--help
Print (on the standard output) a description of the command-line
options understood by gcc. If the -v option is also specified then
--help will also be passed on to the various processes invoked by
gcc, so that they can display the command-line options they accept.
If the -Wextra option has also been specified (prior to the --help
option), then command-line options that have no documentation
associated with them will also be displayed.
--target-help
Print (on the standard output) a description of target-specific
command-line options for each tool. For some targets extra target-
specific information may also be printed.
--help={class|[^]qualifier}[,...]
Print (on the standard output) a description of the command-line
options understood by the compiler that fit into all specified
classes and qualifiers. These are the supported classes:
optimizers
This will display all of the optimization options supported by
the compiler.
warnings
This will display all of the options controlling warning
messages produced by the compiler.
target
This will display target-specific options. Unlike the
--target-help option however, target-specific options of the
linker and assembler will not be displayed. This is because
those tools do not currently support the extended --help=
syntax.
params
This will display the values recognized by the --param option.
language
This will display the options supported for language, where
language is the name of one of the languages supported in this
version of GCC.
common
This will display the options that are common to all languages.
These are the supported qualifiers:
undocumented
Display only those options that are undocumented.
joined
Display options taking an argument that appears after an equal
sign in the same continuous piece of text, such as:
--help=target.
separate
Display options taking an argument that appears as a separate
word following the original option, such as: -ooutput-file.
Thus for example to display all the undocumented target-specific
switches supported by the compiler the following can be used:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the ^
character, so for example to display all binary warning options
(i.e., ones that are either on or off and that do not take an
argument) that have a description, use:
--help=warnings,^joined,^undocumented
The argument to --help= should not consist solely of inverted
qualifiers.
Combining several classes is possible, although this usually
restricts the output by so much that there is nothing to display.
One case where it does work however is when one of the classes is
target. So for example to display all the target-specific
optimization options the following can be used:
--help=target,optimizers
The --help= option can be repeated on the command line. Each
successive use will display its requested class of options,
skipping those that have already been displayed.
If the -Q option appears on the command line before the --help=
option, then the descriptive text displayed by --help= is changed.
Instead of describing the displayed options, an indication is given
as to whether the option is enabled, disabled or set to a specific
value (assuming that the compiler knows this at the point where the
--help= option is used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
The output is sensitive to the effects of previous command-line
options, so for example it is possible to find out which
optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizers
Alternatively you can discover which binary optimizations are
enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to /../ or
/./, or make the path absolute when generating a relative prefix.
--version
Display the version number and copyrights of the invoked GCC.
-wrapper
Invoke all subcommands under a wrapper program. The name of the
wrapper program and its parameters are passed as a comma separated
list.
gcc -c t.c -wrapper gdb,--args
This will invoke all subprograms of gcc under gdb--args, thus the
invocation of cc1 will be gdb--argscc1....
-fplugin=name.so
Load the plugin code in file name.so, assumed to be a shared object
to be dlopen'd by the compiler. The base name of the shared object
file is used to identify the plugin for the purposes of argument
parsing (See -fplugin-arg-name-key=value below). Each plugin
should define the callback functions specified in the Plugins API.
-fplugin-arg-name-key=value
Define an argument called key with a value of value for the plugin
called name.
-fdump-ada-spec[-slim]
For C and C++ source and include files, generate corresponding Ada
specs.
-fdump-go-spec=file
For input files in any language, generate corresponding Go
declarations in file. This generates Go "const", "type", "var",
and "func" declarations which may be a useful way to start writing
a Go interface to code written in some other language.
@file
Read command-line options from file. The options read are inserted
in place of the original @file option. If file does not exist, or
cannot be read, then the option will be treated literally, and not
removed.
Options in file are separated by whitespace. A whitespace
character may be included in an option by surrounding the entire
option in either single or double quotes. Any character (including
a backslash) may be included by prefixing the character to be
included with a backslash. The file may itself contain additional
@file options; any such options will be processed recursively.
CompilingC++Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp,
.CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or
(for shared template code) .tcc; and preprocessed C++ files use the
suffix .ii. GCC recognizes files with these names and compiles them as
C++ programs even if you call the compiler the same way as for
compiling C programs (usually with the name gcc).
However, the use of gcc does not add the C++ library. g++ is a program
that calls GCC and treats .c, .h and .i files as C++ source files
instead of C source files unless -x is used, and automatically
specifies linking against the C++ library. This program is also useful
when precompiling a C header file with a .h extension for use in C++
compilations. On many systems, g++ is also installed with the name
c++.
When you compile C++ programs, you may specify many of the same
command-line options that you use for compiling programs in any
language; or command-line options meaningful for C and related
languages; or options that are meaningful only for C++ programs.
OptionsControllingCDialect
The following options control the dialect of C (or languages derived
from C, such as C++, Objective-C and Objective-C++) that the compiler
accepts:
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is
equivalent to -std=c++98.
This turns off certain features of GCC that are incompatible with
ISO C90 (when compiling C code), or of standard C++ (when compiling
C++ code), such as the "asm" and "typeof" keywords, and predefined
macros such as "unix" and "vax" that identify the type of system
you are using. It also enables the undesirable and rarely used ISO
trigraph feature. For the C compiler, it disables recognition of
C++ style // comments as well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and
"__typeof__" continue to work despite -ansi. You would not want to
use them in an ISO C program, of course, but it is useful to put
them in header files that might be included in compilations done
with -ansi. Alternate predefined macros such as "__unix__" and
"__vax__" are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected
gratuitously. For that, -pedantic is required in addition to
-ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is
used. Some header files may notice this macro and refrain from
declaring certain functions or defining certain macros that the ISO
standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
Functions that would normally be built in but do not have semantics
defined by ISO C (such as "alloca" and "ffs") are not built-in
functions when -ansi is used.
-std=
Determine the language standard. This option is currently only
supported when compiling C or C++.
The compiler can accept several base standards, such as c90 or
c++98, and GNU dialects of those standards, such as gnu90 or
gnu++98. By specifying a base standard, the compiler will accept
all programs following that standard and those using GNU extensions
that do not contradict it. For example, -std=c90 turns off certain
features of GCC that are incompatible with ISO C90, such as the
"asm" and "typeof" keywords, but not other GNU extensions that do
not have a meaning in ISO C90, such as omitting the middle term of
a "?:" expression. On the other hand, by specifying a GNU dialect
of a standard, all features the compiler support are enabled, even
when those features change the meaning of the base standard and
some strict-conforming programs may be rejected. The particular
standard is used by -pedantic to identify which features are GNU
extensions given that version of the standard. For example
-std=gnu90-pedantic would warn about C++ style // comments, while
-std=gnu99-pedantic would not.
A value for this option must be provided; possible values are
c90c89iso9899:1990
Support all ISO C90 programs (certain GNU extensions that
conflict with ISO C90 are disabled). Same as -ansi for C code.
iso9899:199409
ISO C90 as modified in amendment 1.
c99c9xiso9899:1999iso9899:199x
ISO C99. Note that this standard is not yet fully supported;
see <http://gcc.gnu.org/gcc-4.7/c99status.html> for more
information. The names c9x and iso9899:199x are deprecated.
c11c1xiso9899:2011
ISO C11, the 2011 revision of the ISO C standard. Support is
incomplete and experimental. The name c1x is deprecated.
gnu90gnu89
GNU dialect of ISO C90 (including some C99 features). This is
the default for C code.
gnu99gnu9x
GNU dialect of ISO C99. When ISO C99 is fully implemented in
GCC, this will become the default. The name gnu9x is
deprecated.
gnu11gnu1x
GNU dialect of ISO C11. Support is incomplete and
experimental. The name gnu1x is deprecated.
c++98
The 1998 ISO C++ standard plus amendments. Same as -ansi for
C++ code.
gnu++98
GNU dialect of -std=c++98. This is the default for C++ code.
c++11
The 2011 ISO C++ standard plus amendments. Support for C++11
is still experimental, and may change in incompatible ways in
future releases.
gnu++11
GNU dialect of -std=c++11. Support for C++11 is still
experimental, and may change in incompatible ways in future
releases.
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU
semantics for "inline" functions when in C99 mode.
This option is accepted and ignored by GCC versions 4.1.3 up to
but not including 4.3. In GCC versions 4.3 and later it changes
the behavior of GCC in C99 mode. Using this option is roughly
equivalent to adding the "gnu_inline" function attribute to all
inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99
semantics for "inline" when in C99 or gnu99 mode (i.e., it
specifies the default behavior). This option was first supported
in GCC 4.3. This option is not supported in -std=c90 or -std=gnu90
mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and
"__GNUC_STDC_INLINE__" may be used to check which semantics are in
effect for "inline" functions.
-aux-infofilename
Output to the given filename prototyped declarations for all
functions declared and/or defined in a translation unit, including
those in header files. This option is silently ignored in any
language other than C.
Besides declarations, the file indicates, in comments, the origin
of each declaration (source file and line), whether the declaration
was implicit, prototyped or unprototyped (I, N for new or O for
old, respectively, in the first character after the line number and
the colon), and whether it came from a declaration or a definition
(C or F, respectively, in the following character). In the case of
function definitions, a K&R-style list of arguments followed by
their declarations is also provided, inside comments, after the
declaration.
-fallow-parameterless-variadic-functions
Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very
useful as it is not possible to read the arguments. This is only
supported for C as this construct is allowed by C++.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that
code can use these words as identifiers. You can use the keywords
"__asm__", "__inline__" and "__typeof__" instead. -ansi implies
-fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm"
and "inline" are standard keywords. You may want to use the
-fno-gnu-keywords flag instead, which has the same effect. In C99
mode (-std=c99 or -std=gnu99), this switch only affects the "asm"
and "typeof" keywords, since "inline" is a standard keyword in ISO
C99.
-fno-builtin-fno-builtin-function
Don't recognize built-in functions that do not begin with
__builtin_ as prefix.
GCC normally generates special code to handle certain built-in
functions more efficiently; for instance, calls to "alloca" may
become single instructions which adjust the stack directly, and
calls to "memcpy" may become inline copy loops. The resulting code
is often both smaller and faster, but since the function calls no
longer appear as such, you cannot set a breakpoint on those calls,
nor can you change the behavior of the functions by linking with a
different library. In addition, when a function is recognized as a
built-in function, GCC may use information about that function to
warn about problems with calls to that function, or to generate
more efficient code, even if the resulting code still contains
calls to that function. For example, warnings are given with
-Wformat for bad calls to "printf", when "printf" is built in, and
"strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function
function is disabled. function must not begin with __builtin_. If
a function is named that is not built-in in this version of GCC,
this option is ignored. There is no corresponding
-fbuiltin-function option; if you wish to enable built-in functions
selectively when using -fno-builtin or -ffreestanding, you may
define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
Assert that compilation takes place in a hosted environment. This
implies -fbuiltin. A hosted environment is one in which the entire
standard library is available, and in which "main" has a return
type of "int". Examples are nearly everything except a kernel.
This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation takes place in a freestanding environment.
This implies -fno-builtin. A freestanding environment is one in
which the standard library may not exist, and program startup may
not necessarily be at "main". The most obvious example is an OS
kernel. This is equivalent to -fno-hosted.
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++ and
"!$omp" in Fortran. When -fopenmp is specified, the compiler
generates parallel code according to the OpenMP Application Program
Interface v3.0 <http://www.openmp.org/>. This option implies
-pthread, and thus is only supported on targets that have support
for -pthread.
-fgnu-tm
When the option -fgnu-tm is specified, the compiler will generate
code for the Linux variant of Intel's current Transactional Memory
ABI specification document (Revision 1.1, May 6 2009). This is an
experimental feature whose interface may change in future versions
of GCC, as the official specification changes. Please note that
not all architectures are supported for this feature.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with
non-call exceptions (-fnon-call-exceptions).
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
In C++ code, this allows member names in structures to be similar
to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of unnamed fields in structures and unions are only
accepted with this option.
-fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to
structures with anonymous fields to functions that expect pointers
to elements of the type of the field, and permits referring to
anonymous fields declared using a typedef. This is only
supported for C, not C++.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for
strict ISO C conformance) implies -trigraphs.
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling.
This option allows a user supplied "cc1", "cc1plus", or "cc1obj"
via the -B option. The user supplied compilation step can then add
in an additional preprocessing step after normal preprocessing but
before compiling. The default is to use the integrated cpp
(internal cpp)
The semantics of this option will change if "cc1", "cc1plus", and
"cc1obj" are merged.
-traditional-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-
standard C compiler. They are now only supported with the -E
switch. The preprocessor continues to support a pre-standard mode.
See the GNU CPP manual for details.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second
and third arguments. The value of such an expression is void.
This option is not supported for C++.
-flax-vector-conversions
Allow implicit conversions between vectors with differing numbers
of elements and/or incompatible element types. This option should
not be used for new code.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It
is either like "unsigned char" by default or like "signed char" by
default.
Ideally, a portable program should always use "signed char" or
"unsigned char" when it depends on the signedness of an object.
But many programs have been written to use plain "char" and expect
it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let
you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed
char" or "unsigned char", even though its behavior is always just
like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the
negative form of -funsigned-char. Likewise, the option
-fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields-funsigned-bitfields-fno-signed-bitfields-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned,
when the declaration does not use either "signed" or "unsigned".
By default, such a bit-field is signed, because this is consistent:
the basic integer types such as "int" are signed types.
OptionsControllingC++Dialect
This section describes the command-line options that are only
meaningful for C++ programs; but you can also use most of the GNU
compiler options regardless of what language your program is in. For
example, you might compile a file "firstClass.C" like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs;
you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. Version 2 is the version of the C++
ABI that first appeared in G++ 3.4. Version 1 is the version of
the C++ ABI that first appeared in G++ 3.2. Version 0 will always
be the version that conforms most closely to the C++ ABI
specification. Therefore, the ABI obtained using version 0 will
change as ABI bugs are fixed.
The default is version 2.
Version 3 corrects an error in mangling a constant address as a
template argument.
Version 4, which first appeared in G++ 4.5, implements a standard
mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling
of attribute const/volatile on function pointer types, decltype of
a plain decl, and use of a function parameter in the declaration of
another parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion
behavior of C++11 scoped enums and the mangling of template
argument packs, const/static_cast, prefix ++ and --, and a class
scope function used as a template argument.
See also -Wabi.
-fno-access-control
Turn off all access checking. This switch is mainly useful for
working around bugs in the access control code.
-fcheck-new
Check that the pointer returned by "operator new" is non-null
before attempting to modify the storage allocated. This check is
normally unnecessary because the C++ standard specifies that
"operator new" will only return 0 if it is declared tthhrrooww(()), in
which case the compiler will always check the return value even
without this option. In all other cases, when "operator new" has a
non-empty exception specification, memory exhaustion is signalled
by throwing "std::bad_alloc". See also new(nothrow).
-fconserve-space
Put uninitialized or run-time-initialized global variables into the
common segment, as C does. This saves space in the executable at
the cost of not diagnosing duplicate definitions. If you compile
with this flag and your program mysteriously crashes after "main()"
has completed, you may have an object that is being destroyed twice
because two definitions were merged.
This option is no longer useful on most targets, now that support
has been added for putting variables into BSS without making them
common.
-fconstexpr-depth=n
Set the maximum nested evaluation depth for C++11 constexpr
functions to n. A limit is needed to detect endless recursion
during constant expression evaluation. The minimum specified by
the standard is 512.
-fdeduce-init-list
Enable deduction of a template type parameter as
std::initializer_list from a brace-enclosed initializer list, i.e.
template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}
This deduction was implemented as a possible extension to the
originally proposed semantics for the C++11 standard, but was not
part of the final standard, so it is disabled by default. This
option is deprecated, and may be removed in a future version of
G++.
-ffriend-injection
Inject friend functions into the enclosing namespace, so that they
are visible outside the scope of the class in which they are
declared. Friend functions were documented to work this way in the
old Annotated C++ Reference Manual, and versions of G++ before 4.1
always worked that way. However, in ISO C++ a friend function that
is not declared in an enclosing scope can only be found using
argument dependent lookup. This option causes friends to be
injected as they were in earlier releases.
This option is for compatibility, and may be removed in a future
release of G++.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a
temporary that is only used to initialize another object of the
same type. Specifying this option disables that optimization, and
forces G++ to call the copy constructor in all cases.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception
specifications at run time. This option violates the C++ standard,
but may be useful for reducing code size in production builds, much
like defining NDEBUG. This does not give user code permission to
throw exceptions in violation of the exception specifications; the
compiler will still optimize based on the specifications, so
throwing an unexpected exception will result in undefined behavior.
-ffor-scope-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a
for-init-statement is limited to the for loop itself, as specified
by the C++ standard. If -fno-for-scope is specified, the scope of
variables declared in a for-init-statement extends to the end of
the enclosing scope, as was the case in old versions of G++, and
other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to
allow and give a warning for old-style code that would otherwise be
invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this
word as an identifier. You can use the keyword "__typeof__"
instead. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates that are instantiated
implicitly (i.e. by use); only emit code for explicit
instantiations.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates,
either. The default is to handle inlines differently so that
compiles with and without optimization will need the same set of
explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions
controlled by #pragmaimplementation. This will cause linker
errors if these functions are not inlined everywhere they are
called.
-fms-extensions
Disable pedantic warnings about constructs used in MFC, such as
implicit int and getting a pointer to member function via non-
standard syntax.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by
ANSI/ISO C. These include "ffs", "alloca", "_exit", "index",
"bzero", "conjf", and other related functions.
-fnothrow-opt
Treat a "throw()" exception specification as though it were a
"noexcept" specification to reduce or eliminate the text size
overhead relative to a function with no exception specification.
If the function has local variables of types with non-trivial
destructors, the exception specification will actually make the
function smaller because the EH cleanups for those variables can be
optimized away. The semantic effect is that an exception thrown
out of a function with such an exception specification will result
in a call to "terminate" rather than "unexpected".
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor",
"compl", "not", "or" and "xor" as synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need
to issue. Currently, the only such diagnostic issued by G++ is the
one for a name having multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to
warnings. Thus, using -fpermissive will allow some nonconforming
code to compile.
-fno-pretty-templates
When an error message refers to a specialization of a function
template, the compiler will normally print the signature of the
template followed by the template arguments and any typedefs or
typenames in the signature (e.g. "void f(T) [with T = int]" rather
than "void f(int)") so that it's clear which template is involved.
When an error message refers to a specialization of a class
template, the compiler will omit any template arguments that match
the default template arguments for that template. If either of
these behaviors make it harder to understand the error message
rather than easier, using -fno-pretty-templates will disable them.
-frepo
Enable automatic template instantiation at link time. This option
also implies -fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual
functions for use by the C++ run-time type identification features
(dynamic_cast and typeid). If you don't use those parts of the
language, you can save some space by using this flag. Note that
exception handling uses the same information, but it will generate
it as needed. The dynamic_cast operator can still be used for casts
that do not require run-time type information, i.e. casts to "void
*" or to unambiguous base classes.
-fstats
Emit statistics about front-end processing at the end of the
compilation. This information is generally only useful to the G++
development team.
-fstrict-enums
Allow the compiler to optimize using the assumption that a value of
enumerated type can only be one of the values of the enumeration
(as defined in the C++ standard; basically, a value that can be
represented in the minimum number of bits needed to represent all
the enumerators). This assumption may not be valid if the program
uses a cast to convert an arbitrary integer value to the enumerated
type.
-ftemplate-depth=n
Set the maximum instantiation depth for template classes to n. A
limit on the template instantiation depth is needed to detect
endless recursions during template class instantiation. ANSI/ISO
C++ conforming programs must not rely on a maximum depth greater
than 17 (changed to 1024 in C++11). The default value is 900, as
the compiler can run out of stack space before hitting 1024 in some
situations.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++
ABI for thread-safe initialization of local statics. You can use
this option to reduce code size slightly in code that doesn't need
to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with
the "__cxa_atexit" function rather than the "atexit" function.
This option is required for fully standards-compliant handling of
static destructors, but will only work if your C library supports
"__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This will
cause "std::uncaught_exception" to be incorrect, but is necessary
if the runtime routine is not available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to compare
pointers to inline functions or methods where the addresses of the
two functions were taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline
methods with "__attribute__ ((visibility ("hidden")))" so that they
do not appear in the export table of a DSO and do not require a PLT
indirection when used within the DSO. Enabling this option can
have a dramatic effect on load and link times of a DSO as it
massively reduces the size of the dynamic export table when the
library makes heavy use of templates.
The behavior of this switch is not quite the same as marking the
methods as hidden directly, because it does not affect static
variables local to the function or cause the compiler to deduce
that the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate
the effect of the switch for that method. For example, if you do
want to compare pointers to a particular inline method, you might
mark it as having default visibility. Marking the enclosing class
with explicit visibility will have no effect.
Explicitly instantiated inline methods are unaffected by this
option as their linkage might otherwise cross a shared library
boundary.
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's C++
linkage model compatible with that of Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
1. It sets the default visibility to "hidden", like
-fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit
visibility specifications that are defined in more than one
different shared object: those declarations are permitted if
they would have been permitted when this option was not used.
In new code it is better to use -fvisibility=hidden and export
those classes that are intended to be externally visible.
Unfortunately it is possible for code to rely, perhaps
accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static data
members of the same type with the same name but defined in
different shared objects will be different, so changing one will
not change the other; and that pointers to function members defined
in different shared objects may not compare equal. When this flag
is given, it is a violation of the ODR to define types with the
same name differently.
-fno-weak
Do not use weak symbol support, even if it is provided by the
linker. By default, G++ will use weak symbols if they are
available. This option exists only for testing, and should not be
used by end-users; it will result in inferior code and has no
benefits. This option may be removed in a future release of G++.
-nostdinc++
Do not search for header files in the standard directories specific
to C++, but do still search the other standard directories. (This
option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline
functions; they just won't be inlined by default.
-Wabi (C, Objective-C, C++ and Objective-C++ only)
Warn when G++ generates code that is probably not compatible with
the vendor-neutral C++ ABI. Although an effort has been made to
warn about all such cases, there are probably some cases that are
not warned about, even though G++ is generating incompatible code.
There may also be cases where warnings are emitted even though the
code that is generated will be compatible.
You should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not be
binary compatible with code generated by other compilers.
The known incompatibilities in -fabi-version=2 (the default)
include:
o A template with a non-type template parameter of reference type
is mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This is fixed in -fabi-version=3.
o SIMD vector types declared using "__attribute ((vector_size))"
are mangled in a non-standard way that does not allow for
overloading of functions taking vectors of different sizes.
The mangling is changed in -fabi-version=4.
The known incompatibilities in -fabi-version=1 include:
o Incorrect handling of tail-padding for bit-fields. G++ may
attempt to pack data into the same byte as a base class. For
example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte
as"A::f1"; other compilers will not. You can avoid this
problem by explicitly padding "A" so that its size is a
multiple of the byte size on your platform; that will cause G++
and other compilers to layout "B" identically.
o Incorrect handling of tail-padding for virtual bases. G++ does
not use tail padding when laying out virtual bases. For
example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for
"A"; other compilers will. You can avoid this problem by
explicitly padding "A" so that its size is a multiple of its
alignment (ignoring virtual base classes); that will cause G++
and other compilers to layout "C" identically.
o Incorrect handling of bit-fields with declared widths greater
than that of their underlying types, when the bit-fields appear
in a union. For example:
union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make
the union too small by the number of bits in an "int".
o Empty classes can be placed at incorrect offsets. For example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset;
it should be placed at offset zero. G++ mistakenly believes
that the "A" data member of "B" is already at offset zero.
o Names of template functions whose types involve "typename" or
template template parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
It also warns psABI related changes. The known psABI changes at
this point include:
o For SYSV/x86-64, when passing union with long double, it is
changed to pass in memory as specified in psABI. For example:
union U {
long double ld;
int i;
};
"union U" will always be passed in memory.
-Wctor-dtor-privacy (C++ and Objective-C++ only)
Warn when a class seems unusable because all the constructors or
destructors in that class are private, and it has neither friends
nor public static member functions.
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
Warn when delete is used to destroy an instance of a class that has
virtual functions and non-virtual destructor. It is unsafe to
delete an instance of a derived class through a pointer to a base
class if the base class does not have a virtual destructor. This
warning is enabled by -Wall.
-Wnarrowing (C++ and Objective-C++ only)
Warn when a narrowing conversion prohibited by C++11 occurs within
{}, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
With -std=c++11, -Wno-narrowing suppresses the diagnostic required
by the standard. Note that this does not affect the meaning of
well-formed code; narrowing conversions are still considered ill-
formed in SFINAE context.
-Wnoexcept (C++ and Objective-C++ only)
Warn when a noexcept-expression evaluates to false because of a
call to a function that does not have a non-throwing exception
specification (i.e. tthhrrooww(()) or noexcept) but is known by the
compiler to never throw an exception.
-Wnon-virtual-dtor (C++ and Objective-C++ only)
Warn when a class has virtual functions and accessible non-virtual
destructor, in which case it would be possible but unsafe to delete
an instance of a derived class through a pointer to the base class.
This warning is also enabled if -Weffc++ is specified.
-Wreorder (C++ and Objective-C++ only)
Warn when the order of member initializers given in the code does
not match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler will rearrange the member initializers for i and j to
match the declaration order of the members, emitting a warning to
that effect. This warning is enabled by -Wall.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ and Objective-C++ only)
Warn about violations of the following style guidelines from Scott
Meyers' EffectiveC++,SecondEdition book:
o Item 11: Define a copy constructor and an assignment operator
for classes with dynamically allocated memory.
o Item 12: Prefer initialization to assignment in constructors.
o Item 14: Make destructors virtual in base classes.
o Item 15: Have "operator=" return a reference to *this.
o Item 23: Don't try to return a reference when you must return
an object.
Also warn about violations of the following style guidelines from
Scott Meyers' MoreEffectiveC++ book:
o Item 6: Distinguish between prefix and postfix forms of
increment and decrement operators.
o Item 7: Never overload "&&", "||", or ",".
When selecting this option, be aware that the standard library
headers do not obey all of these guidelines; use grep-v to filter
out those warnings.
-Wstrict-null-sentinel (C++ and Objective-C++ only)
Warn also about the use of an uncasted "NULL" as sentinel. When
compiling only with GCC this is a valid sentinel, as "NULL" is
defined to "__null". Although it is a null pointer constant not a
null pointer, it is guaranteed to be of the same size as a pointer.
But this use is not portable across different compilers.
-Wno-non-template-friend (C++ and Objective-C++ only)
Disable warnings when non-templatized friend functions are declared
within a template. Since the advent of explicit template
specification support in G++, if the name of the friend is an
unqualified-id (i.e., friendfoo(int)), the C++ language
specification demands that the friend declare or define an
ordinary, nontemplate function. (Section 14.5.3). Before G++
implemented explicit specification, unqualified-ids could be
interpreted as a particular specialization of a templatized
function. Because this non-conforming behavior is no longer the
default behavior for G++, -Wnon-template-friend allows the compiler
to check existing code for potential trouble spots and is on by
default. This new compiler behavior can be turned off with
-Wno-non-template-friend, which keeps the conformant compiler code
but disables the helpful warning.
-Wold-style-cast (C++ and Objective-C++ only)
Warn if an old-style (C-style) cast to a non-void type is used
within a C++ program. The new-style casts (dynamic_cast,
static_cast, reinterpret_cast, and const_cast) are less vulnerable
to unintended effects and much easier to search for.
-Woverloaded-virtual (C++ and Objective-C++ only)
Warn when a function declaration hides virtual functions from a
base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
will fail to compile.
-Wno-pmf-conversions (C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to member
function to a plain pointer.
-Wsign-promo (C++ and Objective-C++ only)
Warn when overload resolution chooses a promotion from unsigned or
enumerated type to a signed type, over a conversion to an unsigned
type of the same size. Previous versions of G++ would try to
preserve unsignedness, but the standard mandates the current
behavior.
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A&operator=(constA&);, while cfront will use the user-defined operator=.
OptionsControllingObjective-CandObjective-C++Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++
languages themselves.
This section describes the command-line options that are only
meaningful for Objective-C and Objective-C++ programs, but you can also
use most of the language-independent GNU compiler options. For
example, you might compile a file "some_class.m" like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C
and Objective-C++ programs; you can use the other options with any
language supported by GCC.
Note that since Objective-C is an extension of the C language,
Objective-C compilations may also use options specific to the C front-
end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may
use C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and
Objective-C++ programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each
literal string specified with the syntax "@"..."". The default
class name is "NXConstantString" if the GNU runtime is being used,
and "NSConstantString" if the NeXT runtime is being used (see
below). The -fconstant-cfstrings option, if also present, will
override the -fconstant-string-class setting and cause "@"...""
literals to be laid out as constant CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C
runtime. This is the default for most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the
default for NeXT-based systems, including Darwin and Mac OS X. The
macro "__NEXT_RUNTIME__" is predefined if (and only if) this option
is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches ("[receiver
message:arg]") in this translation unit ensure that the receiver is
not "nil". This allows for more efficient entry points in the
runtime to be used. This option is only available in conjunction
with the NeXT runtime and ABI version 0 or 1.
-fobjc-abi-version=n
Use version n of the Objective-C ABI for the selected runtime.
This option is currently supported only for the NeXT runtime. In
that case, Version 0 is the traditional (32-bit) ABI without
support for properties and other Objective-C 2.0 additions.
Version 1 is the traditional (32-bit) ABI with support for
properties and other Objective-C 2.0 additions. Version 2 is the
modern (64-bit) ABI. If nothing is specified, the default is
Version 0 on 32-bit target machines, and Version 2 on 64-bit target
machines.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables
is a C++ object with a non-trivial default constructor. If so,
synthesize a special "- (id) .cxx_construct" instance method which
will run non-trivial default constructors on any such instance
variables, in order, and then return "self". Similarly, check if
any instance variable is a C++ object with a non-trivial
destructor, and if so, synthesize a special "- (void)
.cxx_destruct" method which will run all such default destructors,
in reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct" methods
thusly generated will only operate on instance variables declared
in the current Objective-C class, and not those inherited from
superclasses. It is the responsibility of the Objective-C runtime
to invoke all such methods in an object's inheritance hierarchy.
The "- (id) .cxx_construct" methods will be invoked by the runtime
immediately after a new object instance is allocated; the "- (void)
.cxx_destruct" methods will be invoked immediately before the
runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and
later has support for invoking the "- (id) .cxx_construct" and "-
(void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is
accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++ and Java. This
option is required to use the Objective-C keywords @try, @throw,
@catch, @finally and @synchronized. This option is available with
both the GNU runtime and the NeXT runtime (but not available in
conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++
programs. This option is only available with the NeXT runtime; the
GNU runtime has a different garbage collection implementation that
does not require special compiler flags.
-fobjc-nilcheck
For the NeXT runtime with version 2 of the ABI, check for a nil
receiver in method invocations before doing the actual method call.
This is the default and can be disabled using -fno-objc-nilcheck.
Class methods and super calls are never checked for nil in this way
no matter what this flag is set to. Currently this flag does
nothing when the GNU runtime, or an older version of the NeXT
runtime ABI, is used.
-fobjc-std=objc1
Conform to the language syntax of Objective-C 1.0, the language
recognized by GCC 4.0. This only affects the Objective-C additions
to the C/C++ language; it does not affect conformance to C/C++
standards, which is controlled by the separate C/C++ dialect option
flags. When this option is used with the Objective-C or
Objective-C++ compiler, any Objective-C syntax that is not
recognized by GCC 4.0 is rejected. This is useful if you need to
make sure that your Objective-C code can be compiled with older
versions of GCC.
-freplace-objc-classes
Emit a special marker instructing lldd(1) not to statically link in
the resulting object file, and allow ddyylldd(1) to load it in at run
time instead. This is used in conjunction with the Fix-and-
Continue debugging mode, where the object file in question may be
recompiled and dynamically reloaded in the course of program
execution, without the need to restart the program itself.
Currently, Fix-and-Continue functionality is only available in
conjunction with the NeXT runtime on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily
replaces calls to "objc_getClass("...")" (when the name of the
class is known at compile time) with static class references that
get initialized at load time, which improves run-time performance.
Specifying the -fzero-link flag suppresses this behavior and causes
calls to "objc_getClass("...")" to be retained. This is useful in
Zero-Link debugging mode, since it allows for individual class
implementations to be modified during program execution. The GNU
runtime currently always retains calls to "objc_get_class("...")"
regardless of command-line options.
-gen-decls
Dump interface declarations for all classes seen in the source file
to a file named sourcename.decl.
-Wassign-intercept (Objective-C and Objective-C++ only)
Warn whenever an Objective-C assignment is being intercepted by the
garbage collector.
-Wno-protocol (Objective-C and Objective-C++ only)
If a class is declared to implement a protocol, a warning is issued
for every method in the protocol that is not implemented by the
class. The default behavior is to issue a warning for every method
not explicitly implemented in the class, even if a method
implementation is inherited from the superclass. If you use the
-Wno-protocol option, then methods inherited from the superclass
are considered to be implemented, and no warning is issued for
them.
-Wselector (Objective-C and Objective-C++ only)
Warn if multiple methods of different types for the same selector
are found during compilation. The check is performed on the list
of methods in the final stage of compilation. Additionally, a
check is performed for each selector appearing in a
"@selector(...)" expression, and a corresponding method for that
selector has been found during compilation. Because these checks
scan the method table only at the end of compilation, these
warnings are not produced if the final stage of compilation is not
reached, for example because an error is found during compilation,
or because the -fsyntax-only option is being used.
-Wstrict-selector-match (Objective-C and Objective-C++ only)
Warn if multiple methods with differing argument and/or return
types are found for a given selector when attempting to send a
message using this selector to a receiver of type "id" or "Class".
When this flag is off (which is the default behavior), the compiler
will omit such warnings if any differences found are confined to
types that share the same size and alignment.
-Wundeclared-selector (Objective-C and Objective-C++ only)
Warn if a "@selector(...)" expression referring to an undeclared
selector is found. A selector is considered undeclared if no
method with that name has been declared before the "@selector(...)"
expression, either explicitly in an @interface or @protocol
declaration, or implicitly in an @implementation section. This
option always performs its checks as soon as a "@selector(...)"
expression is found, while -Wselector only performs its checks in
the final stage of compilation. This also enforces the coding
style convention that methods and selectors must be declared before
being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed
by value, if any.
OptionstoControlDiagnosticMessagesFormatting
Traditionally, diagnostic messages have been formatted irrespective of
the output device's aspect (e.g. its width, ...). The options
described below can be used to control the diagnostic messages
formatting algorithm, e.g. how many characters per line, how often
source location information should be reported. Right now, only the
C++ front end can honor these options. However it is expected, in the
near future, that the remaining front ends would be able to digest them
correctly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n
characters. The default is 72 characters for g++ and 0 for the
rest of the front ends supported by GCC. If n is zero, then no
line-wrapping will be done; each error message will appear on a
single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit once source location information; that
is, in case the message is too long to fit on a single physical
line and has to be wrapped, the source location won't be emitted
(as prefix) again, over and over, in subsequent continuation lines.
This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit the same source location information (as
prefix) for physical lines that result from the process of breaking
a message which is too long to fit on a single line.
-fno-diagnostics-show-option
By default, each diagnostic emitted includes text indicating the
command-line option that directly controls the diagnostic (if such
an option is known to the diagnostic machinery). Specifying the
-fno-diagnostics-show-option flag suppresses that behavior.
OptionstoRequestorSuppressWarnings
Warnings are diagnostic messages that report constructions that are not
inherently erroneous but that are risky or suggest there may have been
an error.
The following language-independent options do not enable specific
warnings but control the kinds of diagnostics produced by GCC.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond
that.
-fmax-errors=n
Limits the maximum number of error messages to n, at which point
GCC bails out rather than attempting to continue processing the
source code. If n is 0 (the default), there is no limit on the
number of error messages produced. If -Wfatal-errors is also
specified, then -Wfatal-errors takes precedence over this option.
-w Inhibit all warning messages.
-Werror
Make all warnings into errors.
-Werror=
Make the specified warning into an error. The specifier for a
warning is appended, for example -Werror=switch turns the warnings
controlled by -Wswitch into errors. This switch takes a negative
form, to be used to negate -Werror for specific warnings, for
example -Wno-error=switch makes -Wswitch warnings not be errors,
even when -Werror is in effect.
The warning message for each controllable warning includes the
option that controls the warning. That option can then be used
with -Werror= and -Wno-error= as described above. (Printing of the
option in the warning message can be disabled using the
-fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo.
However, -Wno-error=foo does not imply anything.
-Wfatal-errors
This option causes the compiler to abort compilation on the first
error occurred rather than trying to keep going and printing
further error messages.
You can request many specific warnings with options beginning -W, for
example -Wimplicit to request warnings on implicit declarations. Each
of these specific warning options also has a negative form beginning
-Wno- to turn off warnings; for example, -Wno-implicit. This manual
lists only one of the two forms, whichever is not the default. For
further, language-specific options also refer to C++DialectOptions
and Objective-CandObjective-C++DialectOptions.
When an unrecognized warning option is requested (e.g.,
-Wunknown-warning), GCC will emit a diagnostic stating that the option
is not recognized. However, if the -Wno- form is used, the behavior is
slightly different: No diagnostic will be produced for
-Wno-unknown-warning unless other diagnostics are being produced. This
allows the use of new -Wno- options with old compilers, but if
something goes wrong, the compiler will warn that an unrecognized
option was used.
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject
all programs that use forbidden extensions, and some other programs
that do not follow ISO C and ISO C++. For ISO C, follows the
version of the ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or
without this option (though a rare few will require -ansi or a -std
option specifying the required version of ISO C). However, without
this option, certain GNU extensions and traditional C and C++
features are supported as well. With this option, they are
rejected.
-pedantic does not cause warning messages for use of the alternate
keywords whose names begin and end with __. Pedantic warnings are
also disabled in the expression that follows "__extension__".
However, only system header files should use these escape routes;
application programs should avoid them.
Some users try to use -pedantic to check programs for strict ISO C
conformance. They soon find that it does not do quite what they
want: it finds some non-ISO practices, but not all---only those for
which ISO C requires a diagnostic, and some others for which
diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful
in some instances, but would require considerable additional work
and would be quite different from -pedantic. We don't have plans
to support such a feature in the near future.
Where the standard specified with -std represents a GNU extended
dialect of C, such as gnu90 or gnu99, there is a corresponding basestandard, the version of ISO C on which the GNU extended dialect is
based. Warnings from -pedantic are given where they are required
by the base standard. (It would not make sense for such warnings
to be given only for features not in the specified GNU C dialect,
since by definition the GNU dialects of C include all features the
compiler supports with the given option, and there would be nothing
to warn about.)
-pedantic-errors
Like -pedantic, except that errors are produced rather than
warnings.
-Wall
This enables all the warnings about constructions that some users
consider questionable, and that are easy to avoid (or modify to
prevent the warning), even in conjunction with macros. This also
enables some language-specific warnings described in C++DialectOptions and Objective-CandObjective-C++DialectOptions.
-Wall turns on the following warning flags:
-Waddress-Warray-bounds (only with -O2) -Wc++11-compat-Wchar-subscripts-Wenum-compare (in C/Objc; this is on by default
in C++) -Wimplicit-int (C and Objective-C only)
-Wimplicit-function-declaration (C and Objective-C only) -Wcomment-Wformat-Wmain (only for C/ObjC and unless -ffreestanding)
-Wmaybe-uninitialized-Wmissing-braces-Wnonnull-Wparentheses-Wpointer-sign-Wreorder-Wreturn-type-Wsequence-point-Wsign-compare (only in C++) -Wstrict-aliasing-Wstrict-overflow=1-Wswitch-Wtrigraphs-Wuninitialized-Wunknown-pragmas-Wunused-function-Wunused-label-Wunused-value-Wunused-variable-Wvolatile-register-var
Note that some warning flags are not implied by -Wall. Some of
them warn about constructions that users generally do not consider
questionable, but which occasionally you might wish to check for;
others warn about constructions that are necessary or hard to avoid
in some cases, and there is no simple way to modify the code to
suppress the warning. Some of them are enabled by -Wextra but many
of them must be enabled individually.
-Wextra
This enables some extra warning flags that are not enabled by
-Wall. (This option used to be called -W. The older name is still
supported, but the newer name is more descriptive.)
-Wclobbered-Wempty-body-Wignored-qualifiers-Wmissing-field-initializers-Wmissing-parameter-type (C only)
-Wold-style-declaration (C only) -Woverride-init-Wsign-compare-Wtype-limits-Wuninitialized-Wunused-parameter (only with
-Wunused or -Wall) -Wunused-but-set-parameter (only with -Wunused
or -Wall)
The option -Wextra also prints warning messages for the following
cases:
o A pointer is compared against integer zero with <, <=, >, or
>=.
o (C++ only) An enumerator and a non-enumerator both appear in a
conditional expression.
o (C++ only) Ambiguous virtual bases.
o (C++ only) Subscripting an array that has been declared
register.
o (C++ only) Taking the address of a variable that has been
declared register.
o (C++ only) A base class is not initialized in a derived class'
copy constructor.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause
of error, as programmers often forget that this type is signed on
some machines. This warning is enabled by -Wall.
-Wcomment
Warn whenever a comment-start sequence /* appears in a /* comment,
or whenever a Backslash-Newline appears in a // comment. This
warning is enabled by -Wall.
-Wno-coverage-mismatch
Warn if feedback profiles do not match when using the -fprofile-use
option. If a source file was changed between -fprofile-gen and
-fprofile-use, the files with the profile feedback can fail to
match the source file and GCC cannot use the profile feedback
information. By default, this warning is enabled and is treated as
an error. -Wno-coverage-mismatch can be used to disable the
warning or -Wno-error=coverage-mismatch can be used to disable the
error. Disabling the error for this warning can result in poorly
optimized code and is useful only in the case of very minor changes
such as bug fixes to an existing code-base. Completely disabling
the warning is not recommended.
-Wno-cpp
(C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning" directives.
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Give a warning when a value of type "float" is implicitly promoted
to "double". CPUs with a 32-bit "single-precision" floating-point
unit implement "float" in hardware, but emulate "double" in
software. On such a machine, doing computations using "double"
values is much more expensive because of the overhead required for
software emulation.
It is easy to accidentally do computations with "double" because
floating-point literals are implicitly of type "double". For
example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
the compiler will perform the entire computation with "double"
because the floating-point literal is a "double".
-Wformat
Check calls to "printf" and "scanf", etc., to make sure that the
arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string
make sense. This includes standard functions, and others specified
by format attributes, in the "printf", "scanf", "strftime" and
"strfmon" (an X/Open extension, not in the C standard) families (or
other target-specific families). Which functions are checked
without format attributes having been specified depends on the
standard version selected, and such checks of functions without the
attribute specified are disabled by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by
GNU libc version 2.2. These include all ISO C90 and C99 features,
as well as features from the Single Unix Specification and some BSD
and GNU extensions. Other library implementations may not support
all these features; GCC does not support warning about features
that go beyond a particular library's limitations. However, if
-pedantic is used with -Wformat, warnings will be given about
format features not in the selected standard version (but not for
"strfmon" formats, since those are not in any version of the C
standard).
Since -Wformat also checks for null format arguments for several
functions, -Wformat also implies -Wnonnull.
-Wformat is included in -Wall. For more control over some aspects
of format checking, the options -Wformat-y2k,
-Wno-format-extra-args, -Wno-format-zero-length,
-Wformat-nonliteral, -Wformat-security, and -Wformat=2 are
available, but are not included in -Wall.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats that
may yield only a two-digit year.
-Wno-format-contains-nul
If -Wformat is specified, do not warn about format strings that
contain NUL bytes.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a
"printf" or "scanf" format function. The C standard specifies that
such arguments are ignored.
Where the unused arguments lie between used arguments that are
specified with $ operand number specifications, normally warnings
are still given, since the implementation could not know what type
to pass to "va_arg" to skip the unused arguments. However, in the
case of "scanf" formats, this option will suppress the warning if
the unused arguments are all pointers, since the Single Unix
Specification says that such unused arguments are allowed.
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats.
The C standard specifies that zero-length formats are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a
string literal and so cannot be checked, unless the format function
takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions
that represent possible security problems. At present, this warns
about calls to "printf" and "scanf" functions where the format
string is not a string literal and there are no format arguments,
as in "printf (foo);". This may be a security hole if the format
string came from untrusted input and contains %n. (This is
currently a subset of what -Wformat-nonliteral warns about, but in
future warnings may be added to -Wformat-security that are not
included in -Wformat-nonliteral.)
-Wformat=2
Enable -Wformat plus format checks not included in -Wformat.
Currently equivalent to -Wformat-Wformat-nonliteral-Wformat-security-Wformat-y2k.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring
a non-null value by the "nonnull" function attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled
with the -Wno-nonnull option.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables that are initialized with
themselves. Note this option can only be used with the
-Wuninitialized option.
For example, GCC will warn about "i" being uninitialized in the
following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
-Wimplicit-int (C and Objective-C only)
Warn when a declaration does not specify a type. This warning is
enabled by -Wall.
-Wimplicit-function-declaration (C and Objective-C only)
Give a warning whenever a function is used before being declared.
In C99 mode (-std=c99 or -std=gnu99), this warning is enabled by
default and it is made into an error by -pedantic-errors. This
warning is also enabled by -Wall.
-Wimplicit (C and Objective-C only)
Same as -Wimplicit-int and -Wimplicit-function-declaration. This
warning is enabled by -Wall.
-Wignored-qualifiers (C and C++ only)
Warn if the return type of a function has a type qualifier such as
"const". For ISO C such a type qualifier has no effect, since the
value returned by a function is not an lvalue. For C++, the
warning is only emitted for scalar types or "void". ISO C
prohibits qualified "void" return types on function definitions, so
such return types always receive a warning even without this
option.
This warning is also enabled by -Wextra.
-Wmain
Warn if the type of main is suspicious. main should be a function
with external linkage, returning int, taking either zero arguments,
two, or three arguments of appropriate types. This warning is
enabled by default in C++ and is enabled by either -Wall or
-pedantic.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed.
In the following example, the initializer for a is not fully
bracketed, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when
there is an assignment in a context where a truth value is
expected, or when operators are nested whose precedence people
often get confused about.
Also warn if a comparison like x<=y<=z appears; this is equivalent
to (x<=y?1:0)<=z, which is a different interpretation from
that of ordinary mathematical notation.
Also warn about constructions where there may be confusion to which
"if" statement an "else" branch belongs. Here is an example of
such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C/C++, every "else" branch belongs to the innermost possible
"if" statement, which in this example is "if (b)". This is often
not what the programmer expected, as illustrated in the above
example by indentation the programmer chose. When there is the
potential for this confusion, GCC will issue a warning when this
flag is specified. To eliminate the warning, add explicit braces
around the innermost "if" statement so there is no way the "else"
could belong to the enclosing "if". The resulting code would look
like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
Also warn for dangerous uses of the ?: with omitted middle operand
GNU extension. When the condition in the ?: operator is a boolean
expression the omitted value will be always 1. Often the user
expects it to be a value computed inside the conditional expression
instead.
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of
violations of sequence point rules in the C and C++ standards.
The C and C++ standards defines the order in which expressions in a
C/C++ program are evaluated in terms of sequencepoints, which
represent a partial ordering between the execution of parts of the
program: those executed before the sequence point, and those
executed after it. These occur after the evaluation of a full
expression (one which is not part of a larger expression), after
the evaluation of the first operand of a "&&", "||", "? :" or ","
(comma) operator, before a function is called (but after the
evaluation of its arguments and the expression denoting the called
function), and in certain other places. Other than as expressed by
the sequence point rules, the order of evaluation of subexpressions
of an expression is not specified. All these rules describe only a
partial order rather than a total order, since, for example, if two
functions are called within one expression with no sequence point
between them, the order in which the functions are called is not
specified. However, the standards committee have ruled that
function calls do not overlap.
It is not specified when between sequence points modifications to
the values of objects take effect. Programs whose behavior depends
on this have undefined behavior; the C and C++ standards specify
that "Between the previous and next sequence point an object shall
have its stored value modified at most once by the evaluation of an
expression. Furthermore, the prior value shall be read only to
determine the value to be stored.". If a program breaks these
rules, the results on any particular implementation are entirely
unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] =
b[n++]" and "a[i++] = i;". Some more complicated cases are not
diagnosed by this option, and it may give an occasional false
positive result, but in general it has been found fairly effective
at detecting this sort of problem in programs.
The standard is worded confusingly, therefore there is some debate
over the precise meaning of the sequence point rules in subtle
cases. Links to discussions of the problem, including proposed
formal definitions, may be found on the GCC readings page, at
<http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++.
-Wreturn-type
Warn whenever a function is defined with a return-type that
defaults to "int". Also warn about any "return" statement with no
return-value in a function whose return-type is not "void" (falling
off the end of the function body is considered returning without a
value), and about a "return" statement with an expression in a
function whose return-type is "void".
For C++, a function without return type always produces a
diagnostic message, even when -Wno-return-type is specified. The
only exceptions are main and functions defined in system headers.
This warning is enabled by -Wall.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type
and lacks a "case" for one or more of the named codes of that
enumeration. (The presence of a "default" label prevents this
warning.) "case" labels outside the enumeration range also provoke
warnings when this option is used (even if there is a "default"
label). This warning is enabled by -Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type
and lacks a "case" for one or more of the named codes of that
enumeration. "case" labels outside the enumeration range also
provoke warnings when this option is used. The only difference
between -Wswitch and this option is that this option gives a
warning about an omitted enumeration code even if there is a
"default" label.
-Wsync-nand (C and C++ only)
Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch"
built-in functions are used. These functions changed semantics in
GCC 4.4.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning
of the program (trigraphs within comments are not warned about).
This warning is enabled by -Wall.
-Wunused-but-set-parameter
Warn whenever a function parameter is assigned to, but otherwise
unused (aside from its declaration).
To suppress this warning use the unused attribute.
This warning is also enabled by -Wunused together with -Wextra.
-Wunused-but-set-variable
Warn whenever a local variable is assigned to, but otherwise unused
(aside from its declaration). This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
This warning is also enabled by -Wunused, which is enabled by
-Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a
non-inline static function is unused. This warning is enabled by
-Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is
enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
Warn when a typedef locally defined in a function is not used.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its
declaration.
To suppress this warning use the unused attribute.
-Wno-unused-result
Do not warn if a caller of a function marked with attribute
"warn_unused_result" does not use its return value. The default is
-Wunused-result.
-Wunused-variable
Warn whenever a local variable or non-constant static variable is
unused aside from its declaration. This warning is enabled by
-Wall.
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not
used. To suppress this warning cast the unused expression to void.
This includes an expression-statement or the left-hand side of a
comma expression that contains no side effects. For example, an
expression such as x[i,j] will cause a warning, while x[(void)i,j]
will not.
This warning is enabled by -Wall.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you
must either specify -Wextra-Wunused (note that -Wall implies
-Wunused), or separately specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being
initialized or if a variable may be clobbered by a "setjmp" call.
In C++, warn if a non-static reference or non-static const member
appears in a class without constructors.
If you want to warn about code that uses the uninitialized value of
the variable in its own initializer, use the -Winit-self option.
These warnings occur for individual uninitialized or clobbered
elements of structure, union or array variables as well as for
variables that are uninitialized or clobbered as a whole. They do
not occur for variables or elements declared "volatile". Because
these warnings depend on optimization, the exact variables or
elements for which there are warnings will depend on the precise
optimization options and version of GCC used.
Note that there may be no warning about a variable that is used
only to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the
warnings are printed.
-Wmaybe-uninitialized
For an automatic variable, if there exists a path from the function
entry to a use of the variable that is initialized, but there exist
some other paths the variable is not initialized, the compiler will
emit a warning if it can not prove the uninitialized paths do not
happen at run time. These warnings are made optional because GCC is
not smart enough to see all the reasons why the code might be
correct despite appearing to have an error. Here is one example of
how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always
initialized, but GCC doesn't know this. To suppress the warning,
the user needs to provide a default case with assert(0) or similar
code.
This option also warns when a non-volatile automatic variable might
be changed by a call to "longjmp". These warnings as well are
possible only in optimizing compilation.
The compiler sees only the calls to "setjmp". It cannot know where
"longjmp" will be called; in fact, a signal handler could call it
at any point in the code. As a result, you may get a warning even
when there is in fact no problem because "longjmp" cannot in fact
be called at the place that would cause a problem.
Some spurious warnings can be avoided if you declare all the
functions you use that never return as "noreturn".
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas
Warn when a "#pragma" directive is encountered that is not
understood by GCC. If this command-line option is used, warnings
will even be issued for unknown pragmas in system header files.
This is not the case if the warnings were only enabled by the -Wall
command-line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters,
invalid syntax, or conflicts between pragmas. See also
-Wunknown-pragmas.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It
warns about code that might break the strict aliasing rules that
the compiler is using for optimization. The warning does not catch
all cases, but does attempt to catch the more common pitfalls. It
is included in -Wall. It is equivalent to -Wstrict-aliasing=3-Wstrict-aliasing=n
This option is only active when -fstrict-aliasing is active. It
warns about code that might break the strict aliasing rules that
the compiler is using for optimization. Higher levels correspond
to higher accuracy (fewer false positives). Higher levels also
correspond to more effort, similar to the way -O works.
-Wstrict-aliasing is equivalent to -Wstrict-aliasing=n, with n=3.
Level 1: Most aggressive, quick, least accurate. Possibly useful
when higher levels do not warn but -fstrict-aliasing still breaks
the code, as it has very few false negatives. However, it has many
false positives. Warns for all pointer conversions between
possibly incompatible types, even if never dereferenced. Runs in
the front end only.
Level 2: Aggressive, quick, not too precise. May still have many
false positives (not as many as level 1 though), and few false
negatives (but possibly more than level 1). Unlike level 1, it
only warns when an address is taken. Warns about incomplete types.
Runs in the front end only.
Level 3 (default for -Wstrict-aliasing): Should have very few false
positives and few false negatives. Slightly slower than levels 1
or 2 when optimization is enabled. Takes care of the common
pun+dereference pattern in the front end: "*(int*)&some_float". If
optimization is enabled, it also runs in the back end, where it
deals with multiple statement cases using flow-sensitive points-to
information. Only warns when the converted pointer is
dereferenced. Does not warn about incomplete types.
-Wstrict-overflow-Wstrict-overflow=n
This option is only active when -fstrict-overflow is active. It
warns about cases where the compiler optimizes based on the
assumption that signed overflow does not occur. Note that it does
not warn about all cases where the code might overflow: it only
warns about cases where the compiler implements some optimization.
Thus this warning depends on the optimization level.
An optimization that assumes that signed overflow does not occur is
perfectly safe if the values of the variables involved are such
that overflow never does, in fact, occur. Therefore this warning
can easily give a false positive: a warning about code that is not
actually a problem. To help focus on important issues, several
warning levels are defined. No warnings are issued for the use of
undefined signed overflow when estimating how many iterations a
loop will require, in particular when determining whether a loop
will be executed at all.
-Wstrict-overflow=1
Warn about cases that are both questionable and easy to avoid.
For example: "x + 1 > x"; with -fstrict-overflow, the compiler
will simplify this to 1. This level of -Wstrict-overflow is
enabled by -Wall; higher levels are not, and must be explicitly
requested.
-Wstrict-overflow=2
Also warn about other cases where a comparison is simplified to
a constant. For example: "abs (x) >= 0". This can only be
simplified when -fstrict-overflow is in effect, because "abs
(INT_MIN)" overflows to "INT_MIN", which is less than zero.
-Wstrict-overflow (with no level) is the same as
-Wstrict-overflow=2.
-Wstrict-overflow=3
Also warn about other cases where a comparison is simplified.
For example: "x + 1 > 1" will be simplified to "x > 0".
-Wstrict-overflow=4
Also warn about other simplifications not covered by the above
cases. For example: "(x * 10) / 5" will be simplified to "x *
2".
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the magnitude
of a constant involved in a comparison. For example: "x + 2 >
y" will be simplified to "x + 1 >= y". This is reported only
at the highest warning level because this simplification
applies to many comparisons, so this warning level will give a
very large number of false positives.
-Wsuggest-attribute=[pure|const|noreturn]
Warn for cases where adding an attribute may be beneficial. The
attributes currently supported are listed below.
-Wsuggest-attribute=pure-Wsuggest-attribute=const-Wsuggest-attribute=noreturn
Warn about functions that might be candidates for attributes
"pure", "const" or "noreturn". The compiler only warns for
functions visible in other compilation units or (in the case of
"pure" and "const") if it cannot prove that the function
returns normally. A function returns normally if it doesn't
contain an infinite loop nor returns abnormally by throwing,
calling "abort()" or trapping. This analysis requires option
-fipa-pure-const, which is enabled by default at -O and higher.
Higher optimization levels improve the accuracy of the
analysis.
-Warray-bounds
This option is only active when -ftree-vrp is active (default for
-O2 and above). It warns about subscripts to arrays that are always
out of bounds. This warning is enabled by -Wall.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating-
point division by zero is not warned about, as it can be a
legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files.
Warnings from system headers are normally suppressed, on the
assumption that they usually do not indicate real problems and
would only make the compiler output harder to read. Using this
command-line option tells GCC to emit warnings from system headers
as if they occurred in user code. However, note that using -Wall
in conjunction with this option will not warn about unknown pragmas
in system headers---for that, -Wunknown-pragmas must also be used.
-Wtrampolines
Warn about trampolines generated for pointers to nested functions.
A trampoline is a small piece of data or code that is created at run
time on the stack when the address of a nested function is taken, and
is used to call the nested function indirectly. For some targets, it
is made up of data only and thus requires no special treatment. But,
for most targets, it is made up of code and thus requires the stack
to be made executable in order for the program to work properly.
-Wfloat-equal
Warn if floating-point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as approximations to
infinitely precise real numbers. If you are doing this, then you
need to compute (by analyzing the code, or in some other way) the
maximum or likely maximum error that the computation introduces,
and allow for it when performing comparisons (and when producing
output, but that's a different problem). In particular, instead of
testing for equality, you would check to see whether the two values
have ranges that overlap; and this is done with the relational
operators, so equality comparisons are probably mistaken.
-Wtraditional (C and Objective-C only)
Warn about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO C constructs that have
no traditional C equivalent, and/or problematic constructs that
should be avoided.
o Macro parameters that appear within string literals in the
macro body. In traditional C macro replacement takes place
within string literals, but does not in ISO C.
o In traditional C, some preprocessor directives did not exist.
Traditional preprocessors would only consider a line to be a
directive if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C
understands but would ignore because the # does not appear as
the first character on the line. It also suggests you hide
directives like #pragma not understood by traditional C by
indenting them. Some traditional implementations would not
recognize #elif, so it suggests avoiding it altogether.
o A function-like macro that appears without arguments.
o The unary plus operator.
o The U integer constant suffix, or the F or L floating-point
constant suffixes. (Traditional C does support the L suffix on
integer constants.) Note, these suffixes appear in macros
defined in the system headers of most modern systems, e.g. the
_MIN/_MAX macros in "<limits.h>". Use of these macros in user
code might normally lead to spurious warnings, however GCC's
integrated preprocessor has enough context to avoid warning in
these cases.
o A function declared external in one block and then used after
the end of the block.
o A "switch" statement has an operand of type "long".
o A non-"static" function declaration follows a "static" one.
This construct is not accepted by some traditional C compilers.
o The ISO type of an integer constant has a different width or
signedness from its traditional type. This warning is only
issued if the base of the constant is ten. I.e. hexadecimal or
octal values, which typically represent bit patterns, are not
warned about.
o Usage of ISO string concatenation is detected.
o Initialization of automatic aggregates.
o Identifier conflicts with labels. Traditional C lacks a
separate namespace for labels.
o Initialization of unions. If the initializer is zero, the
warning is omitted. This is done under the assumption that the
zero initializer in user code appears conditioned on e.g.
"__STDC__" to avoid missing initializer warnings and relies on
default initialization to zero in the traditional C case.
o Conversions by prototypes between fixed/floating-point values
and vice versa. The absence of these prototypes when compiling
with traditional C would cause serious problems. This is a
subset of the possible conversion warnings, for the full set
use -Wtraditional-conversion.
o Use of ISO C style function definitions. This warning
intentionally is not issued for prototype declarations or
variadic functions because these ISO C features will appear in
your code when using libiberty's traditional C compatibility
macros, "PARAMS" and "VPARAMS". This warning is also bypassed
for nested functions because that feature is already a GCC
extension and thus not relevant to traditional C compatibility.
-Wtraditional-conversion (C and Objective-C only)
Warn if a prototype causes a type conversion that is different from
what would happen to the same argument in the absence of a
prototype. This includes conversions of fixed point to floating
and vice versa, and conversions changing the width or signedness of
a fixed-point argument except when the same as the default
promotion.
-Wdeclaration-after-statement (C and Objective-C only)
Warn when a declaration is found after a statement in a block.
This construct, known from C++, was introduced with ISO C99 and is
by default allowed in GCC. It is not supported by ISO C90 and was
not supported by GCC versions before GCC 3.0.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wno-endif-labels
Do not warn whenever an #else or an #endif are followed by text.
-Wshadow
Warn whenever a local variable or type declaration shadows another
variable, parameter, type, or class member (in C++), or whenever a
built-in function is shadowed. Note that in C++, the compiler will
not warn if a local variable shadows a struct/class/enum, but will
warn if it shadows an explicit typedef.
-Wlarger-than=len
Warn whenever an object of larger than len bytes is defined.
-Wframe-larger-than=len
Warn if the size of a function frame is larger than len bytes. The
computation done to determine the stack frame size is approximate
and not conservative. The actual requirements may be somewhat
greater than len even if you do not get a warning. In addition,
any space allocated via "alloca", variable-length arrays, or
related constructs is not included by the compiler when determining
whether or not to issue a warning.
-Wno-free-nonheap-object
Do not warn when attempting to free an object that was not
allocated on the heap.
-Wstack-usage=len
Warn if the stack usage of a function might be larger than len
bytes. The computation done to determine the stack usage is
conservative. Any space allocated via "alloca", variable-length
arrays, or related constructs is included by the compiler when
determining whether or not to issue a warning.
The message is in keeping with the output of -fstack-usage.
o If the stack usage is fully static but exceeds the specified
amount, it's:
warning: stack usage is 1120 bytes
o If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
o If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler could not
assume anything on the bounds of the loop indices. With
-funsafe-loop-optimizations warn if the compiler made such
assumptions.
-Wno-pedantic-ms-format (MinGW targets only)
Disables the warnings about non-ISO "printf" / "scanf" format width
specifiers "I32", "I64", and "I" used on Windows targets depending
on the MS runtime, when you are using the options -Wformat and
-pedantic without gnu-extensions.
-Wpointer-arith
Warn about anything that depends on the "size of" a function type
or of "void". GNU C assigns these types a size of 1, for
convenience in calculations with "void *" pointers and pointers to
functions. In C++, warn also when an arithmetic operation involves
"NULL". This warning is also enabled by -pedantic.
-Wtype-limits
Warn if a comparison is always true or always false due to the
limited range of the data type, but do not warn for constant
expressions. For example, warn if an unsigned variable is compared
against zero with < or >=. This warning is also enabled by
-Wextra.
-Wbad-function-cast (C and Objective-C only)
Warn whenever a function call is cast to a non-matching type. For
example, warn if "int malloc()" is cast to "anything *".
-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the common subset
of ISO C and ISO C++, e.g. request for implicit conversion from
"void *" to a pointer to non-"void" type.
-Wc++11-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++
1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are
keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is
enabled by -Wall.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier
from the target type. For example, warn if a "const char *" is
cast to an ordinary "char *".
Also warn when making a cast that introduces a type qualifier in an
unsafe way. For example, casting "char **" to "const char **" is
unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of
the target is increased. For example, warn if a "char *" is cast
to an "int *" on machines where integers can only be accessed at
two- or four-byte boundaries.
-Wwrite-strings
When compiling C, give string constants the type "const
char[length]" so that copying the address of one into a non-"const"
"char *" pointer will get a warning. These warnings will help you
find at compile time code that can try to write into a string
constant, but only if you have been very careful about using
"const" in declarations and prototypes. Otherwise, it will just be
a nuisance. This is why we did not make -Wall request these
warnings.
When compiling C++, warn about the deprecated conversion from
string literals to "char *". This warning is enabled by default
for C++ programs.
-Wclobbered
Warn for variables that might be changed by longjmp or vfork. This
warning is also enabled by -Wextra.
-Wconversion
Warn for implicit conversions that may alter a value. This includes
conversions between real and integer, like "abs (x)" when "x" is
"double"; conversions between signed and unsigned, like "unsigned
ui = -1"; and conversions to smaller types, like "sqrtf (M_PI)". Do
not warn for explicit casts like "abs ((int) x)" and "ui =
(unsigned) -1", or if the value is not changed by the conversion
like in "abs (2.0)". Warnings about conversions between signed and
unsigned integers can be disabled by using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-
defined conversions; and conversions that will never use a type
conversion operator: conversions to "void", the same type, a base
class or a reference to them. Warnings about conversions between
signed and unsigned integers are disabled by default in C++ unless
-Wsign-conversion is explicitly enabled.
-Wno-conversion-null (C++ and Objective-C++ only)
Do not warn for conversions between "NULL" and non-pointer types.
-Wconversion-null is enabled by default.
-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
Warn when a literal '0' is used as null pointer constant. This can
be useful to facilitate the conversion to "nullptr" in C++11.
-Wempty-body
Warn if an empty body occurs in an if, else or dowhile statement.
This warning is also enabled by -Wextra.
-Wenum-compare
Warn about a comparison between values of different enumerated
types. In C++ enumeral mismatches in conditional expressions are
also diagnosed and the warning is enabled by default. In C this
warning is enabled by -Wall.
-Wjump-misses-init (C, Objective-C only)
Warn if a "goto" statement or a "switch" statement jumps forward
across the initialization of a variable, or jumps backward to a
label after the variable has been initialized. This only warns
about variables that are initialized when they are declared. This
warning is only supported for C and Objective-C; in C++ this sort
of branch is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. It can be disabled
with the -Wno-jump-misses-init option.
-Wsign-compare
Warn when a comparison between signed and unsigned values could
produce an incorrect result when the signed value is converted to
unsigned. This warning is also enabled by -Wextra; to get the
other warnings of -Wextra without this warning, use -Wextra-Wno-sign-compare.
-Wsign-conversion
Warn for implicit conversions that may change the sign of an
integer value, like assigning a signed integer expression to an
unsigned integer variable. An explicit cast silences the warning.
In C, this option is enabled also by -Wconversion.
-Waddress
Warn about suspicious uses of memory addresses. These include using
the address of a function in a conditional expression, such as
"void func(void); if (func)", and comparisons against the memory
address of a string literal, such as "if (x == "abc")". Such uses
typically indicate a programmer error: the address of a function
always evaluates to true, so their use in a conditional usually
indicate that the programmer forgot the parentheses in a function
call; and comparisons against string literals result in unspecified
behavior and are not portable in C, so they usually indicate that
the programmer intended to use "strcmp". This warning is enabled
by -Wall.
-Wlogical-op
Warn about suspicious uses of logical operators in expressions.
This includes using logical operators in contexts where a bit-wise
operator is likely to be expected.
-Waggregate-return
Warn if any functions that return structures or unions are defined
or called. (In languages where you can return an array, this also
elicits a warning.)
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as
unrecognized attributes, function attributes applied to variables,
etc. This will not stop errors for incorrect use of supported
attributes.
-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined. This
suppresses warnings for redefinition of "__TIMESTAMP__",
"__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__".
-Wstrict-prototypes (C and Objective-C only)
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if preceded by a declaration that specifies the
argument types.)
-Wold-style-declaration (C and Objective-C only)
Warn for obsolescent usages, according to the C Standard, in a
declaration. For example, warn if storage-class specifiers like
"static" are not the first things in a declaration. This warning
is also enabled by -Wextra.
-Wold-style-definition (C and Objective-C only)
Warn if an old-style function definition is used. A warning is
given even if there is a previous prototype.
-Wmissing-parameter-type (C and Objective-C only)
A function parameter is declared without a type specifier in
K&R-style functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
-Wmissing-prototypes (C and Objective-C only)
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. The aim is to detect global functions that
are not declared in header files.
-Wmissing-declarations
Warn if a global function is defined without a previous
declaration. Do so even if the definition itself provides a
prototype. Use this option to detect global functions that are not
declared in header files. In C++, no warnings are issued for
function templates, or for inline functions, or for functions in
anonymous namespaces.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For
example, the following code would cause such a warning, because
"x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the
following modification would not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This warning is included in -Wextra. To get other -Wextra warnings
without this one, use -Wextra-Wno-missing-field-initializers.
-Wmissing-format-attribute
Warn about function pointers that might be candidates for "format"
attributes. Note these are only possible candidates, not absolute
ones. GCC will guess that function pointers with "format"
attributes that are used in assignment, initialization, parameter
passing or return statements should have a corresponding "format"
attribute in the resulting type. I.e. the left-hand side of the
assignment or initialization, the type of the parameter variable,
or the return type of the containing function respectively should
also have a "format" attribute to avoid the warning.
GCC will also warn about function definitions that might be
candidates for "format" attributes. Again, these are only possible
candidates. GCC will guess that "format" attributes might be
appropriate for any function that calls a function like "vprintf"
or "vscanf", but this might not always be the case, and some
functions for which "format" attributes are appropriate may not be
detected.
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually
they indicate a typo in the user's code, as they have
implementation-defined values, and should not be used in portable
code.
-Wnormalized=<none|id|nfc|nfkc>
In ISO C and ISO C++, two identifiers are different if they are
different sequences of characters. However, sometimes when
characters outside the basic ASCII character set are used, you can
have two different character sequences that look the same. To
avoid confusion, the ISO 10646 standard sets out some normalizationrules which when applied ensure that two sequences that look the
same are turned into the same sequence. GCC can warn you if you
are using identifiers that have not been normalized; this option
controls that warning.
There are four levels of warning supported by GCC. The default is
-Wnormalized=nfc, which warns about any identifier that is not in
the ISO 10646 "C" normalized form, NFC. NFC is the recommended
form for most uses.
Unfortunately, there are some characters allowed in identifiers by
ISO C and ISO C++ that, when turned into NFC, are not allowed in
identifiers. That is, there's no way to use these symbols in
portable ISO C or C++ and have all your identifiers in NFC.
-Wnormalized=id suppresses the warning for these characters. It is
hoped that future versions of the standards involved will correct
this, which is why this option is not the default.
You can switch the warning off for all characters by writing
-Wnormalized=none. You would only want to do this if you were
using some other normalization scheme (like "D"), because otherwise
you can easily create bugs that are literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look
identical in some fonts or display methodologies, especially once
formatting has been applied. For instance "\u207F", "SUPERSCRIPT
LATIN SMALL LETTER N", will display just like a regular "n" that
has been placed in a superscript. ISO 10646 defines the NFKC
normalization scheme to convert all these into a standard form as
well, and GCC will warn if your code is not in NFKC if you use
-Wnormalized=nfkc. This warning is comparable to warning about
every identifier that contains the letter O because it might be
confused with the digit 0, and so is not the default, but may be
useful as a local coding convention if the programming environment
is unable to be fixed to display these characters distinctly.
-Wno-deprecated
Do not warn about usage of deprecated features.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as
deprecated by using the "deprecated" attribute.
-Wno-overflow
Do not warn about compile-time overflow in constant expressions.
-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is overridden
when using designated initializers.
This warning is included in -Wextra. To get other -Wextra warnings
without this one, use -Wextra-Wno-override-init.
-Wpacked
Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit. For
instance, in this code, the variable "f.x" in "struct bar" will be
misaligned even though "struct bar" does not itself have the packed
attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on
bit-fields of type "char". This has been fixed in GCC 4.4 but the
change can lead to differences in the structure layout. GCC
informs you when the offset of such a field has changed in GCC 4.4.
For example there is no longer a 4-bit padding between field "a"
and "b" in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use
-Wno-packed-bitfield-compat to disable this warning.
-Wpadded
Warn if padding is included in a structure, either to align an
element of the structure or to align the whole structure.
Sometimes when this happens it is possible to rearrange the fields
of the structure to reduce the padding and so make the structure
smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even
in cases where multiple declaration is valid and changes nothing.
-Wnested-externs (C and Objective-C only)
Warn if an "extern" declaration is encountered within a function.
-Winline
Warn if a function can not be inlined and it was declared as
inline. Even with this option, the compiler will not warn about
failures to inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or
not to inline a function. For example, the compiler takes into
account the size of the function being inlined and the amount of
inlining that has already been done in the current function.
Therefore, seemingly insignificant changes in the source program
can cause the warnings produced by -Winline to appear or disappear.
-Wno-invalid-offsetof (C++ and Objective-C++ only)
Suppress warnings from applying the offsetof macro to a non-POD
type. According to the 1998 ISO C++ standard, applying offsetof to
a non-POD type is undefined. In existing C++ implementations,
however, offsetof typically gives meaningful results even when
applied to certain kinds of non-POD types. (Such as a simple struct
that fails to be a POD type only by virtue of having a
constructor.) This flag is for users who are aware that they are
writing nonportable code and who have deliberately chosen to ignore
the warning about it.
The restrictions on offsetof may be relaxed in a future version of
the C++ standard.
-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of a
different size. In C++, casting to a pointer type of smaller size
is an error. Wint-to-pointer-cast is enabled by default.
-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an integer type of a
different size.
-Winvalid-pch
Warn if a precompiled header is found in the search path but can't
be used.
-Wlong-long
Warn if longlong type is used. This is enabled by either
-pedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit
the warning messages, use -Wno-long-long.
-Wvariadic-macros
Warn if variadic macros are used in pedantic ISO C90 mode, or the
GNU alternate syntax when in pedantic ISO C99 mode. This is
default. To inhibit the warning messages, use
-Wno-variadic-macros.
-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD capabilities
of the architecture. Mainly useful for the performance tuning.
Vector operation can be implemented "piecewise", which means that
the scalar operation is performed on every vector element; "in
parallel", which means that the vector operation is implemented
using scalars of wider type, which normally is more performance
efficient; and "as a single scalar", which means that vector fits
into a scalar type.
-Wvla
Warn if variable length array is used in the code. -Wno-vla will
prevent the -pedantic warning of the variable length array.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile
modifier does not inhibit all optimizations that may eliminate
reads and/or writes to register variables. This warning is enabled
by -Wall.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning
does not generally indicate that there is anything wrong with your
code; it merely indicates that GCC's optimizers were unable to
handle the code effectively. Often, the problem is that your code
is too big or too complex; GCC will refuse to optimize programs
when the optimization itself is likely to take inordinate amounts
of time.
-Wpointer-sign (C and Objective-C only)
Warn for pointer argument passing or assignment with different
signedness. This option is only supported for C and Objective-C.
It is implied by -Wall and by -pedantic, which can be disabled with
-Wno-pointer-sign.
-Wstack-protector
This option is only active when -fstack-protector is active. It
warns about functions that will not be protected against stack
smashing.
-Wno-mudflap
Suppress warnings about constructs that cannot be instrumented by
-fmudflap.
-Woverlength-strings
Warn about string constants that are longer than the "minimum
maximum" length specified in the C standard. Modern compilers
generally allow string constants that are much longer than the
standard's minimum limit, but very portable programs should avoid
using longer strings.
The limit applies after string constant concatenation, and does not
count the trailing NUL. In C90, the limit was 509 characters; in
C99, it was raised to 4095. C++98 does not specify a normative
minimum maximum, so we do not diagnose overlength strings in C++.
This option is implied by -pedantic, and can be disabled with
-Wno-overlength-strings.
-Wunsuffixed-float-constants (C and Objective-C only)
GCC will issue a warning for any floating constant that does not
have a suffix. When used together with -Wsystem-headers it will
warn about such constants in system header files. This can be
useful when preparing code to use with the "FLOAT_CONST_DECIMAL64"
pragma from the decimal floating-point extension to C99.
OptionsforDebuggingYourProgramorGCC
GCC has various special options that are used for debugging either your
program or GCC:
-g Produce debugging information in the operating system's native
format (stabs, COFF, XCOFF, or DWARF 2). GDB can work with this
debugging information.
On most systems that use stabs format, -g enables use of extra
debugging information that only GDB can use; this extra information
makes debugging work better in GDB but will probably make other
debuggers crash or refuse to read the program. If you want to
control for certain whether to generate the extra information, use
-gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
GCC allows you to use -g with -O. The shortcuts taken by optimized
code may occasionally produce surprising results: some variables
you declared may not exist at all; flow of control may briefly move
where you did not expect it; some statements may not be executed
because they compute constant results or their values were already
at hand; some statements may execute in different places because
they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This
makes it reasonable to use the optimizer for programs that might
have bugs.
The following options are useful when GCC is generated with the
capability for more than one debugging format.
-ggdb
Produce debugging information for use by GDB. This means to use
the most expressive format available (DWARF 2, stabs, or the native
format if neither of those are supported), including GDB extensions
if at all possible.
-gstabs
Produce debugging information in stabs format (if that is
supported), without GDB extensions. This is the format used by DBX
on most BSD systems. On MIPS, Alpha and System V Release 4 systems
this option produces stabs debugging output that is not understood
by DBX or SDB. On System V Release 4 systems this option requires
the GNU assembler.
-feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is
supported), for only symbols that are actually used.
-femit-class-debug-always
Instead of emitting debugging information for a C++ class in only
one object file, emit it in all object files using the class. This
option should be used only with debuggers that are unable to handle
the way GCC normally emits debugging information for classes
because using this option will increase the size of debugging
information by as much as a factor of two.
-fno-debug-types-section
By default when using DWARF v4 or higher type DIEs will be put into
their own .debug_types section instead of making them part of the
.debug_info section. It is more efficient to put them in a
separate comdat sections since the linker will then be able to
remove duplicates. But not all DWARF consumers support
.debug_types sections yet.
-gstabs+
Produce debugging information in stabs format (if that is
supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to make
other debuggers crash or refuse to read the program.
-gcoff
Produce debugging information in COFF format (if that is
supported). This is the format used by SDB on most System V
systems prior to System V Release 4.
-gxcoff
Produce debugging information in XCOFF format (if that is
supported). This is the format used by the DBX debugger on IBM
RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is
supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to make
other debuggers crash or refuse to read the program, and may cause
assemblers other than the GNU assembler (GAS) to fail with an
error.
-gdwarf-version
Produce debugging information in DWARF format (if that is
supported). This is the format used by DBX on IRIX 6. The value
of version may be either 2, 3 or 4; the default version is 2.
Note that with DWARF version 2 some ports require, and will always
use, some non-conflicting DWARF 3 extensions in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for
maximum benefit.
-grecord-gcc-switches
This switch causes the command-line options used to invoke the
compiler that may affect code generation to be appended to the
DW_AT_producer attribute in DWARF debugging information. The
options are concatenated with spaces separating them from each
other and from the compiler version. See also
-frecord-gcc-switches for another way of storing compiler options
into the object file.
-gno-record-gcc-switches
Disallow appending command-line options to the DW_AT_producer
attribute in DWARF debugging information. This is the default.
-gstrict-dwarf
Disallow using extensions of later DWARF standard version than
selected with -gdwarf-version. On most targets using non-
conflicting DWARF extensions from later standard versions is
allowed.
-gno-strict-dwarf
Allow using extensions of later DWARF standard version than
selected with -gdwarf-version.
-gvms
Produce debugging information in VMS debug format (if that is
supported). This is the format used by DEBUG on VMS systems.
-glevel-ggdblevel-gstabslevel-gcofflevel-gxcofflevel-gvmslevel
Request debugging information and also use level to specify how
much information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0 negates
-g.
Level 1 produces minimal information, enough for making backtraces
in parts of the program that you don't plan to debug. This
includes descriptions of functions and external variables, but no
information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro
definitions present in the program. Some debuggers support macro
expansion when you use -g3.
-gdwarf-2 does not accept a concatenated debug level, because GCC
used to support an option -gdwarf that meant to generate debug
information in version 1 of the DWARF format (which is very
different from version 2), and it would have been too confusing.
That debug format is long obsolete, but the option cannot be
changed now. Instead use an additional -glevel option to change
the debug level for DWARF.
-gtoggle
Turn off generation of debug info, if leaving out this option would
have generated it, or turn it on at level 2 otherwise. The
position of this argument in the command line does not matter, it
takes effect after all other options are processed, and it does so
only once, no matter how many times it is given. This is mainly
intended to be used with -fcompare-debug.
-fdump-final-insns[=file]
Dump the final internal representation (RTL) to file. If the
optional argument is omitted (or if file is "."), the name of the
dump file will be determined by appending ".gkd" to the compilation
output file name.
-fcompare-debug[=opts]
If no error occurs during compilation, run the compiler a second
time, adding opts and -fcompare-debug-second to the arguments
passed to the second compilation. Dump the final internal
representation in both compilations, and print an error if they
differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty
and nonzero, implicitly enables -fcompare-debug. If
GCC_COMPARE_DEBUG is defined to a string starting with a dash, then
it is used for opts, otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is
equivalent to -fno-compare-debug, which disables the dumping of the
final representation and the second compilation, preventing even
GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set
GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC
will reject as an invalid option in any actual compilation (rather
than preprocessing, assembly or linking). To get just a warning,
setting GCC_COMPARE_DEBUG to -w%n-fcompare-debugnotoverridden
will do.
-fcompare-debug-second
This option is implicitly passed to the compiler for the second
compilation requested by -fcompare-debug, along with options to
silence warnings, and omitting other options that would cause side-
effect compiler outputs to files or to the standard output. Dump
files and preserved temporary files are renamed so as to contain
the ".gk" additional extension during the second compilation, to
avoid overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the
first compilation to be skipped, which makes it useful for little
other than debugging the compiler proper.
-feliminate-dwarf2-dups
Compress DWARF2 debugging information by eliminating duplicated
information about each symbol. This option only makes sense when
generating DWARF2 debugging information with -gdwarf-2.
-femit-struct-debug-baseonly
Emit debug information for struct-like types only when the base
name of the compilation source file matches the base name of file
in which the struct was defined.
This option substantially reduces the size of debugging
information, but at significant potential loss in type information
to the debugger. See -femit-struct-debug-reduced for a less
aggressive option. See -femit-struct-debug-detailed for more
detailed control.
This option works only with DWARF 2.
-femit-struct-debug-reduced
Emit debug information for struct-like types only when the base
name of the compilation source file matches the base name of file
in which the type was defined, unless the struct is a template or
defined in a system header.
This option significantly reduces the size of debugging
information, with some potential loss in type information to the
debugger. See -femit-struct-debug-baseonly for a more aggressive
option. See -femit-struct-debug-detailed for more detailed
control.
This option works only with DWARF 2.
-femit-struct-debug-detailed[=spec-list]
Specify the struct-like types for which the compiler will generate
debug information. The intent is to reduce duplicate struct debug
information between different object files within the same program.
This option is a detailed version of -femit-struct-debug-reduced
and -femit-struct-debug-baseonly, which will serve for most needs.
A specification has the
syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The optional first word limits the specification to structs that
are used directly (dir:) or used indirectly (ind:). A struct type
is used directly when it is the type of a variable, member.
Indirect uses arise through pointers to structs. That is, when use
of an incomplete struct would be legal, the use is indirect. An
example is structonedirect;structtwo*indirect;.
The optional second word limits the specification to ordinary
structs (ord:) or generic structs (gen:). Generic structs are a
bit complicated to explain. For C++, these are non-explicit
specializations of template classes, or non-template classes within
the above. Other programming languages have generics, but
-femit-struct-debug-detailed does not yet implement them.
The third word specifies the source files for those structs for
which the compiler will emit debug information. The values none
and any have the normal meaning. The value base means that the
base of name of the file in which the type declaration appears must
match the base of the name of the main compilation file. In
practice, this means that types declared in foo.c and foo.h will
have debug information, but types declared in other header will
not. The value sys means those types satisfying base or declared
in system or compiler headers.
You may need to experiment to determine the best settings for your
application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF 2.
-fno-merge-debug-strings
Direct the linker to not merge together strings in the debugging
information that are identical in different object files. Merging
is not supported by all assemblers or linkers. Merging decreases
the size of the debug information in the output file at the cost of
increasing link processing time. Merging is enabled by default.
-fdebug-prefix-map=old=new
When compiling files in directory old, record debugging information
describing them as in new instead.
-fno-dwarf2-cfi-asm
Emit DWARF 2 unwind info as compiler generated ".eh_frame" section
instead of using GAS ".cfi_*" directives.
-p Generate extra code to write profile information suitable for the
analysis program prof. You must use this option when compiling the
source files you want data about, and you must also use it when
linking.
-pg Generate extra code to write profile information suitable for the
analysis program gprof. You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-Q Makes the compiler print out each function name as it is compiled,
and print some statistics about each pass when it finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by
each pass when it finishes.
-fmem-report
Makes the compiler print some statistics about permanent memory
allocation when it finishes.
-fpre-ipa-mem-report-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent memory
allocation before or after interprocedural optimization.
-fstack-usage
Makes the compiler output stack usage information for the program,
on a per-function basis. The filename for the dump is made by
appending .su to the auxname. auxname is generated from the name
of the output file, if explicitly specified and it is not an
executable, otherwise it is the basename of the source file. An
entry is made up of three fields:
o The name of the function.
o A number of bytes.
o One or more qualifiers: "static", "dynamic", "bounded".
The qualifier "static" means that the function manipulates the
stack statically: a fixed number of bytes are allocated for the
frame on function entry and released on function exit; no stack
adjustments are otherwise made in the function. The second field
is this fixed number of bytes.
The qualifier "dynamic" means that the function manipulates the
stack dynamically: in addition to the static allocation described
above, stack adjustments are made in the body of the function, for
example to push/pop arguments around function calls. If the
qualifier "bounded" is also present, the amount of these
adjustments is bounded at compile time and the second field is an
upper bound of the total amount of stack used by the function. If
it is not present, the amount of these adjustments is not bounded
at compile time and the second field only represents the bounded
part.
-fprofile-arcs
Add code so that program flow arcs are instrumented. During
execution the program records how many times each branch and call
is executed and how many times it is taken or returns. When the
compiled program exits it saves this data to a file called
auxname.gcda for each source file. The data may be used for
profile-directed optimizations (-fbranch-probabilities), or for
test coverage analysis (-ftest-coverage). Each object file's
auxname is generated from the name of the output file, if
explicitly specified and it is not the final executable, otherwise
it is the basename of the source file. In both cases any suffix is
removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda
for output file specified as -odir/foo.o).
--coverage
This option is used to compile and link code instrumented for
coverage analysis. The option is a synonym for -fprofile-arcs-ftest-coverage (when compiling) and -lgcov (when linking). See
the documentation for those options for more details.
o Compile the source files with -fprofile-arcs plus optimization
and code generation options. For test coverage analysis, use
the additional -ftest-coverage option. You do not need to
profile every source file in a program.
o Link your object files with -lgcov or -fprofile-arcs (the
latter implies the former).
o Run the program on a representative workload to generate the
arc profile information. This may be repeated any number of
times. You can run concurrent instances of your program, and
provided that the file system supports locking, the data files
will be correctly updated. Also "fork" calls are detected and
correctly handled (double counting will not happen).
o For profile-directed optimizations, compile the source files
again with the same optimization and code generation options
plus -fbranch-probabilities.
o For test coverage analysis, use gcov to produce human readable
information from the .gcno and .gcda files. Refer to the gcov
documentation for further information.
With -fprofile-arcs, for each function of your program GCC creates
a program flow graph, then finds a spanning tree for the graph.
Only arcs that are not on the spanning tree have to be
instrumented: the compiler adds code to count the number of times
that these arcs are executed. When an arc is the only exit or only
entrance to a block, the instrumentation code can be added to the
block; otherwise, a new basic block must be created to hold the
instrumentation code.
-ftest-coverage
Produce a notes file that the gcov code-coverage utility can use to
show program coverage. Each source file's note file is called
auxname.gcno. Refer to the -fprofile-arcs option above for a
description of auxname and instructions on how to generate test
coverage data. Coverage data will match the source files more
closely, if you do not optimize.
-fdbg-cnt-list
Print the name and the counter upper bound for all debug counters.
-fdbg-cnt=counter-value-list
Set the internal debug counter upper bound. counter-value-list is
a comma-separated list of name:value pairs which sets the upper
bound of each debug counter name to value. All debug counters have
the initial upper bound of UINT_MAX, thus dbg_cnt() returns true
always unless the upper bound is set by this option. e.g. With
-fdbg-cnt=dce:10,tail_call:0 dbg_cnt(dce) will return true only for
first 10 invocations
-fenable-kind-pass-fdisable-kind-pass=range-list
This is a set of debugging options that are used to explicitly
disable/enable optimization passes. For compiler users, regular
options for enabling/disabling passes should be used instead.
*<-fdisable-ipa-pass>
Disable ipa pass pass. pass is the pass name. If the same pass
is statically invoked in the compiler multiple times, the pass
name should be appended with a sequential number starting from
1.
*<-fdisable-rtl-pass>
*<-fdisable-rtl-pass=range-list>
Disable rtl pass pass. pass is the pass name. If the same
pass is statically invoked in the compiler multiple times, the
pass name should be appended with a sequential number starting
from 1. range-list is a comma seperated list of function
ranges or assembler names. Each range is a number pair
seperated by a colon. The range is inclusive in both ends. If
the range is trivial, the number pair can be simplified as a
single number. If the function's cgraph node's uid is falling
within one of the specified ranges, the pass is disabled for
that function. The uid is shown in the function header of a
dump file, and the pass names can be dumped by using option
-fdump-passes.
*<-fdisable-tree-pass>
*<-fdisable-tree-pass=range-list>
Disable tree pass pass. See -fdisable-rtl for the description
of option arguments.
*<-fenable-ipa-pass>
Enable ipa pass pass. pass is the pass name. If the same pass
is statically invoked in the compiler multiple times, the pass
name should be appended with a sequential number starting from
1.
*<-fenable-rtl-pass>
*<-fenable-rtl-pass=range-list>
Enable rtl pass pass. See -fdisable-rtl for option argument
description and examples.
*<-fenable-tree-pass>
*<-fenable-tree-pass=range-list>
Enable tree pass pass. See -fdisable-rtl for the description
of option arguments.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
-dletters-fdump-rtl-pass
Says to make debugging dumps during compilation at times specified
by letters. This is used for debugging the RTL-based passes of the
compiler. The file names for most of the dumps are made by
appending a pass number and a word to the dumpname, and the files
are created in the directory of the output file. Note that the
pass number is computed statically as passes get registered into
the pass manager. Thus the numbering is not related to the dynamic
order of execution of passes. In particular, a pass installed by a
plugin could have a number over 200 even if it executed quite
early. dumpname is generated from the name of the output file, if
explicitly specified and it is not an executable, otherwise it is
the basename of the source file. These switches may have different
effects when -E is used for preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some -d
option letters. Here are the possible letters for use in pass and
letters, and their meanings:
-fdump-rtl-alignments
Dump after branch alignments have been computed.
-fdump-rtl-asmcons
Dump after fixing rtl statements that have unsatisfied in/out
constraints.
-fdump-rtl-auto_inc_dec
Dump after auto-inc-dec discovery. This pass is only run on
architectures that have auto inc or auto dec instructions.
-fdump-rtl-barriers
Dump after cleaning up the barrier instructions.
-fdump-rtl-bbpart
Dump after partitioning hot and cold basic blocks.
-fdump-rtl-bbro
Dump after block reordering.
-fdump-rtl-btl1-fdump-rtl-btl2-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the
two branch target load optimization passes.
-fdump-rtl-bypass
Dump after jump bypassing and control flow optimizations.
-fdump-rtl-combine
Dump after the RTL instruction combination pass.
-fdump-rtl-compgotos
Dump after duplicating the computed gotos.
-fdump-rtl-ce1-fdump-rtl-ce2-fdump-rtl-ce3-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable
dumping after the three if conversion passes.
-fdump-rtl-cprop_hardreg
Dump after hard register copy propagation.
-fdump-rtl-csa
Dump after combining stack adjustments.
-fdump-rtl-cse1-fdump-rtl-cse2-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the
two common sub-expression elimination passes.
-fdump-rtl-dce
Dump after the standalone dead code elimination passes.
-fdump-rtl-dbr
Dump after delayed branch scheduling.
-fdump-rtl-dce1-fdump-rtl-dce2-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the
two dead store elimination passes.
-fdump-rtl-eh
Dump after finalization of EH handling code.
-fdump-rtl-eh_ranges
Dump after conversion of EH handling range regions.
-fdump-rtl-expand
Dump after RTL generation.
-fdump-rtl-fwprop1-fdump-rtl-fwprop2-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after
the two forward propagation passes.
-fdump-rtl-gcse1-fdump-rtl-gcse2-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after
global common subexpression elimination.
-fdump-rtl-init-regs
Dump after the initialization of the registers.
-fdump-rtl-initvals
Dump after the computation of the initial value sets.
-fdump-rtl-into_cfglayout
Dump after converting to cfglayout mode.
-fdump-rtl-ira
Dump after iterated register allocation.
-fdump-rtl-jump
Dump after the second jump optimization.
-fdump-rtl-loop2-fdump-rtl-loop2 enables dumping after the rtl loop
optimization passes.
-fdump-rtl-mach
Dump after performing the machine dependent reorganization
pass, if that pass exists.
-fdump-rtl-mode_sw
Dump after removing redundant mode switches.
-fdump-rtl-rnreg
Dump after register renumbering.
-fdump-rtl-outof_cfglayout
Dump after converting from cfglayout mode.
-fdump-rtl-peephole2
Dump after the peephole pass.
-fdump-rtl-postreload
Dump after post-reload optimizations.
-fdump-rtl-pro_and_epilogue
Dump after generating the function prologues and epilogues.
-fdump-rtl-regmove
Dump after the register move pass.
-fdump-rtl-sched1-fdump-rtl-sched2-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after
the basic block scheduling passes.
-fdump-rtl-see
Dump after sign extension elimination.
-fdump-rtl-seqabstr
Dump after common sequence discovery.
-fdump-rtl-shorten
Dump after shortening branches.
-fdump-rtl-sibling
Dump after sibling call optimizations.
-fdump-rtl-split1-fdump-rtl-split2-fdump-rtl-split3-fdump-rtl-split4-fdump-rtl-split5-fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3,
-fdump-rtl-split4 and -fdump-rtl-split5 enable dumping after
five rounds of instruction splitting.
-fdump-rtl-sms
Dump after modulo scheduling. This pass is only run on some
architectures.
-fdump-rtl-stack
Dump after conversion from GCC's "flat register file" registers
to the x87's stack-like registers. This pass is only run on
x86 variants.
-fdump-rtl-subreg1-fdump-rtl-subreg2-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after
the two subreg expansion passes.
-fdump-rtl-unshare
Dump after all rtl has been unshared.
-fdump-rtl-vartrack
Dump after variable tracking.
-fdump-rtl-vregs
Dump after converting virtual registers to hard registers.
-fdump-rtl-web
Dump after live range splitting.
-fdump-rtl-regclass-fdump-rtl-subregs_of_mode_init-fdump-rtl-subregs_of_mode_finish-fdump-rtl-dfinit-fdump-rtl-dfinish
These dumps are defined but always produce empty files.
-da-fdump-rtl-all
Produce all the dumps listed above.
-dA Annotate the assembler output with miscellaneous debugging
information.
-dD Dump all macro definitions, at the end of preprocessing, in
addition to normal output.
-dH Produce a core dump whenever an error occurs.
-dp Annotate the assembler output with a comment indicating which
pattern and alternative was used. The length of each
instruction is also printed.
-dP Dump the RTL in the assembler output as a comment before each
instruction. Also turns on -dp annotation.
-dv For each of the other indicated dump files (-fdump-rtl-pass),
dump a representation of the control flow graph suitable for
viewing with VCG to file.pass.vcg.
-dx Just generate RTL for a function instead of compiling it.
Usually used with -fdump-rtl-expand.
-fdump-noaddr
When doing debugging dumps, suppress address output. This makes it
more feasible to use diff on debugging dumps for compiler
invocations with different compiler binaries and/or different text
/ bss / data / heap / stack / dso start locations.
-fdump-unnumbered
When doing debugging dumps, suppress instruction numbers and
address output. This makes it more feasible to use diff on
debugging dumps for compiler invocations with different options, in
particular with and without -g.
-fdump-unnumbered-links
When doing debugging dumps (see -d option above), suppress
instruction numbers for the links to the previous and next
instructions in a sequence.
-fdump-translation-unit (C++ only)
-fdump-translation-unit-options (C++ only)
Dump a representation of the tree structure for the entire
translation unit to a file. The file name is made by appending .tu
to the source file name, and the file is created in the same
directory as the output file. If the -options form is used,
options controls the details of the dump as described for the
-fdump-tree options.
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
Dump a representation of each class's hierarchy and virtual
function table layout to a file. The file name is made by
appending .class to the source file name, and the file is created
in the same directory as the output file. If the -options form is
used, options controls the details of the dump as described for the
-fdump-tree options.
-fdump-ipa-switch
Control the dumping at various stages of inter-procedural analysis
language tree to a file. The file name is generated by appending a
switch specific suffix to the source file name, and the file is
created in the same directory as the output file. The following
dumps are possible:
all Enables all inter-procedural analysis dumps.
cgraph
Dumps information about call-graph optimization, unused
function removal, and inlining decisions.
inline
Dump after function inlining.
-fdump-passes
Dump the list of optimization passes that are turned on and off by
the current command-line options.
-fdump-statistics-option
Enable and control dumping of pass statistics in a separate file.
The file name is generated by appending a suffix ending in
.statistics to the source file name, and the file is created in the
same directory as the output file. If the -option form is used,
-stats will cause counters to be summed over the whole compilation
unit while -details will dump every event as the passes generate
them. The default with no option is to sum counters for each
function compiled.
-fdump-tree-switch-fdump-tree-switch-options
Control the dumping at various stages of processing the
intermediate language tree to a file. The file name is generated
by appending a switch specific suffix to the source file name, and
the file is created in the same directory as the output file. If
the -options form is used, options is a list of - separated options
which control the details of the dump. Not all options are
applicable to all dumps; those that are not meaningful will be
ignored. The following options are available
address
Print the address of each node. Usually this is not meaningful
as it changes according to the environment and source file.
Its primary use is for tying up a dump file with a debug
environment.
asmname
If "DECL_ASSEMBLER_NAME" has been set for a given decl, use
that in the dump instead of "DECL_NAME". Its primary use is
ease of use working backward from mangled names in the assembly
file.
slim
Inhibit dumping of members of a scope or body of a function
merely because that scope has been reached. Only dump such
items when they are directly reachable by some other path.
When dumping pretty-printed trees, this option inhibits dumping
the bodies of control structures.
raw Print a raw representation of the tree. By default, trees are
pretty-printed into a C-like representation.
details
Enable more detailed dumps (not honored by every dump option).
stats
Enable dumping various statistics about the pass (not honored
by every dump option).
blocks
Enable showing basic block boundaries (disabled in raw dumps).
vops
Enable showing virtual operands for every statement.
lineno
Enable showing line numbers for statements.
uid Enable showing the unique ID ("DECL_UID") for each variable.
verbose
Enable showing the tree dump for each statement.
eh Enable showing the EH region number holding each statement.
scev
Enable showing scalar evolution analysis details.
all Turn on all options, except raw, slim, verbose and lineno.
The following tree dumps are possible:
original
Dump before any tree based optimization, to file.original.
optimized
Dump after all tree based optimization, to file.optimized.
gimple
Dump each function before and after the gimplification pass to
a file. The file name is made by appending .gimple to the
source file name.
cfg Dump the control flow graph of each function to a file. The
file name is made by appending .cfg to the source file name.
vcg Dump the control flow graph of each function to a file in VCG
format. The file name is made by appending .vcg to the source
file name. Note that if the file contains more than one
function, the generated file cannot be used directly by VCG.
You will need to cut and paste each function's graph into its
own separate file first.
ch Dump each function after copying loop headers. The file name
is made by appending .ch to the source file name.
ssa Dump SSA related information to a file. The file name is made
by appending .ssa to the source file name.
alias
Dump aliasing information for each function. The file name is
made by appending .alias to the source file name.
ccp Dump each function after CCP. The file name is made by
appending .ccp to the source file name.
storeccp
Dump each function after STORE-CCP. The file name is made by
appending .storeccp to the source file name.
pre Dump trees after partial redundancy elimination. The file name
is made by appending .pre to the source file name.
fre Dump trees after full redundancy elimination. The file name is
made by appending .fre to the source file name.
copyprop
Dump trees after copy propagation. The file name is made by
appending .copyprop to the source file name.
store_copyprop
Dump trees after store copy-propagation. The file name is made
by appending .store_copyprop to the source file name.
dce Dump each function after dead code elimination. The file name
is made by appending .dce to the source file name.
mudflap
Dump each function after adding mudflap instrumentation. The
file name is made by appending .mudflap to the source file
name.
sra Dump each function after performing scalar replacement of
aggregates. The file name is made by appending .sra to the
source file name.
sink
Dump each function after performing code sinking. The file
name is made by appending .sink to the source file name.
dom Dump each function after applying dominator tree optimizations.
The file name is made by appending .dom to the source file
name.
dse Dump each function after applying dead store elimination. The
file name is made by appending .dse to the source file name.
phiopt
Dump each function after optimizing PHI nodes into straightline
code. The file name is made by appending .phiopt to the source
file name.
forwprop
Dump each function after forward propagating single use
variables. The file name is made by appending .forwprop to the
source file name.
copyrename
Dump each function after applying the copy rename optimization.
The file name is made by appending .copyrename to the source
file name.
nrv Dump each function after applying the named return value
optimization on generic trees. The file name is made by
appending .nrv to the source file name.
vect
Dump each function after applying vectorization of loops. The
file name is made by appending .vect to the source file name.
slp Dump each function after applying vectorization of basic
blocks. The file name is made by appending .slp to the source
file name.
vrp Dump each function after Value Range Propagation (VRP). The
file name is made by appending .vrp to the source file name.
all Enable all the available tree dumps with the flags provided in
this option.
-ftree-vectorizer-verbose=n
This option controls the amount of debugging output the vectorizer
prints. This information is written to standard error, unless
-fdump-tree-all or -fdump-tree-vect is specified, in which case it
is output to the usual dump listing file, .vect. For n=0 no
diagnostic information is reported. If n=1 the vectorizer reports
each loop that got vectorized, and the total number of loops that
got vectorized. If n=2 the vectorizer also reports non-vectorized
loops that passed the first analysis phase (vect_analyze_loop_form)
- i.e. countable, inner-most, single-bb, single-entry/exit loops.
This is the same verbosity level that -fdump-tree-vect-stats uses.
Higher verbosity levels mean either more information dumped for
each reported loop, or same amount of information reported for more
loops: if n=3, vectorizer cost model information is reported. If
n=4, alignment related information is added to the reports. If
n=5, data-references related information (e.g. memory dependences,
memory access-patterns) is added to the reports. If n=6, the
vectorizer reports also non-vectorized inner-most loops that did
not pass the first analysis phase (i.e., may not be countable, or
may have complicated control-flow). If n=7, the vectorizer reports
also non-vectorized nested loops. If n=8, SLP related information
is added to the reports. For n=9, all the information the
vectorizer generates during its analysis and transformation is
reported. This is the same verbosity level that
-fdump-tree-vect-details uses.
-frandom-seed=string
This option provides a seed that GCC uses when it would otherwise
use random numbers. It is used to generate certain symbol names
that have to be different in every compiled file. It is also used
to place unique stamps in coverage data files and the object files
that produce them. You can use the -frandom-seed option to produce
reproducibly identical object files.
The string should be different for every file you compile.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls
the amount of debugging output the scheduler prints. This
information is written to standard error, unless -fdump-rtl-sched1
or -fdump-rtl-sched2 is specified, in which case it is output to
the usual dump listing file, .sched1 or .sched2 respectively.
However for n greater than nine, the output is always printed to
standard error.
For n greater than zero, -fsched-verbose outputs the same
information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n
greater than one, it also output basic block probabilities,
detailed ready list information and unit/insn info. For n greater
than two, it includes RTL at abort point, control-flow and regions
info. And for n over four, -fsched-verbose also includes
dependence info.
-save-temps-save-temps=cwd
Store the usual "temporary" intermediate files permanently; place
them in the current directory and name them based on the source
file. Thus, compiling foo.c with -c-save-temps would produce
files foo.i and foo.s, as well as foo.o. This creates a
preprocessed foo.i output file even though the compiler now
normally uses an integrated preprocessor.
When used in combination with the -x command-line option,
-save-temps is sensible enough to avoid over writing an input
source file with the same extension as an intermediate file. The
corresponding intermediate file may be obtained by renaming the
source file before using -save-temps.
If you invoke GCC in parallel, compiling several different source
files that share a common base name in different subdirectories or
the same source file compiled for multiple output destinations, it
is likely that the different parallel compilers will interfere with
each other, and overwrite the temporary files. For instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&
may result in foo.i and foo.o being written to simultaneously by
both compilers.
-save-temps=obj
Store the usual "temporary" intermediate files permanently. If the
-o option is used, the temporary files are based on the object
file. If the -o option is not used, the -save-temps=obj switch
behaves like -save-temps.
For example:
gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar
would create foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i,
dir2/yfoobar.s, and dir2/yfoobar.o.
-time[=file]
Report the CPU time taken by each subprocess in the compilation
sequence. For C source files, this is the compiler proper and
assembler (plus the linker if linking is done).
Without the specification of an output file, the output looks like
this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that is time
spent executing the program itself. The second number is "system
time", time spent executing operating system routines on behalf of
the program. Both numbers are in seconds.
With the specification of an output file, the output is appended to
the named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>
The "user time" and the "system time" are moved before the program
name, and the options passed to the program are displayed, so that
one can later tell what file was being compiled, and with which
options.
-fvar-tracking
Run variable tracking pass. It computes where variables are stored
at each position in code. Better debugging information is then
generated (if the debugging information format supports this
information).
It is enabled by default when compiling with optimization (-Os, -O,
-O2, ...), debugging information (-g) and the debug info format
supports it.
-fvar-tracking-assignments
Annotate assignments to user variables early in the compilation and
attempt to carry the annotations over throughout the compilation
all the way to the end, in an attempt to improve debug information
while optimizing. Use of -gdwarf-4 is recommended along with it.
It can be enabled even if var-tracking is disabled, in which case
annotations will be created and maintained, but discarded at the
end.
-fvar-tracking-assignments-toggle
Toggle -fvar-tracking-assignments, in the same way that -gtoggle
toggles -g.
-print-file-name=library
Print the full absolute name of the library file library that would
be used when linking---and don't do anything else. With this
option, GCC does not compile or link anything; it just prints the
file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by
any other switches present in the command line. This directory is
supposed to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler
switches that enable them. The directory name is separated from
the switches by ;, and each switch starts with an @ instead of the
-, without spaces between multiple switches. This is supposed to
ease shell-processing.
-print-multi-os-directory
Print the path to OS libraries for the selected multilib, relative
to some lib subdirectory. If OS libraries are present in the lib
subdirectory and no multilibs are used, this is usually just ., if
OS libraries are present in libsuffix sibling directories this
prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are
present in lib/subdir subdirectories it prints e.g. amd64, sparcv9
or ev6.
-print-multiarch
Print the path to OS libraries for the selected multiarch, relative
to some lib subdirectory.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do
want to link with libgcc.a. You can do
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory and a list
of program and library directories gcc will search---and don't do
anything else.
This is useful when gcc prints the error message installationproblem,cannotexeccpp0:Nosuchfileordirectory. To resolve
this you either need to put cpp0 and the other compiler components
where gcc expects to find them, or you can set the environment
variable GCC_EXEC_PREFIX to the directory where you installed them.
Don't forget the trailing /.
-print-sysroot
Print the target sysroot directory that will be used during
compilation. This is the target sysroot specified either at
configure time or using the --sysroot option, possibly with an
extra suffix that depends on compilation options. If no target
sysroot is specified, the option prints nothing.
-print-sysroot-headers-suffix
Print the suffix added to the target sysroot when searching for
headers, or give an error if the compiler is not configured with
such a suffix---and don't do anything else.
-dumpmachine
Print the compiler's target machine (for example,
i686-pc-linux-gnu)---and don't do anything else.
-dumpversion
Print the compiler version (for example, 3.0)---and don't do
anything else.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else.
(This is used when GCC itself is being built.)
-feliminate-unused-debug-types
Normally, when producing DWARF2 output, GCC will emit debugging
information for all types declared in a compilation unit,
regardless of whether or not they are actually used in that
compilation unit. Sometimes this is useful, such as if, in the
debugger, you want to cast a value to a type that is not actually
used in your program (but is declared). More often, however, this
results in a significant amount of wasted space. With this option,
GCC will avoid producing debug symbol output for types that are
nowhere used in the source file being compiled.
OptionsThatControlOptimization
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the
cost of compilation and to make debugging produce the expected results.
Statements are independent: if you stop the program with a breakpoint
between statements, you can then assign a new value to any variable or
change the program counter to any other statement in the function and
get exactly the results you would expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the
performance and/or code size at the expense of compilation time and
possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the
program. Compiling multiple files at once to a single output file mode
allows the compiler to use information gained from all of the files
when compiling each of them.
Not all optimizations are controlled directly by a flag. Only
optimizations that have a flag are listed in this section.
Most optimizations are only enabled if an -O level is set on the
command line. Otherwise they are disabled, even if individual
optimization flags are specified.
Depending on the target and how GCC was configured, a slightly
different set of optimizations may be enabled at each -O level than
those listed here. You can invoke GCC with -Q--help=optimizers to
find out the exact set of optimizations that are enabled at each level.
-O-O1 Optimize. Optimizing compilation takes somewhat more time, and a
lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution time,
without performing any optimizations that take a great deal of
compilation time.
-O turns on the following optimization flags:
-fauto-inc-dec-fcompare-elim-fcprop-registers-fdce-fdefer-pop-fdelayed-branch-fdse-fguess-branch-probability-fif-conversion2-fif-conversion-fipa-pure-const-fipa-profile-fipa-reference-fmerge-constants-fsplit-wide-types-ftree-bit-ccp-ftree-builtin-call-dce-ftree-ccp-ftree-ch-ftree-copyrename-ftree-dce-ftree-dominator-opts-ftree-dse-ftree-forwprop-ftree-fre-ftree-phiprop-ftree-sra-ftree-pta-ftree-ter-funit-at-a-time-O also turns on -fomit-frame-pointer on machines where doing so
does not interfere with debugging.
-O2 Optimize even more. GCC performs nearly all supported
optimizations that do not involve a space-speed tradeoff. As
compared to -O, this option increases both compilation time and the
performance of the generated code.
-O2 turns on all optimization flags specified by -O. It also turns
on the following optimization flags: -fthread-jumps-falign-functions-falign-jumps-falign-loops-falign-labels-fcaller-saves-fcrossjumping-fcse-follow-jumps-fcse-skip-blocks-fdelete-null-pointer-checks-fdevirtualize-fexpensive-optimizations-fgcse-fgcse-lm-finline-small-functions-findirect-inlining-fipa-sra-foptimize-sibling-calls-fpartial-inlining-fpeephole2-fregmove-freorder-blocks-freorder-functions-frerun-cse-after-loop-fsched-interblock-fsched-spec-fschedule-insns-fschedule-insns2-fstrict-aliasing-fstrict-overflow-ftree-switch-conversion-ftree-tail-merge-ftree-pre-ftree-vrp
Please note the warning under -fgcse about invoking -O2 on programs
that use computed gotos.
-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2
and also turns on the -finline-functions, -funswitch-loops,
-fpredictive-commoning, -fgcse-after-reload, -ftree-vectorize and
-fipa-cp-clone options.
-O0 Reduce compilation time and make debugging produce the expected
results. This is the default.
-Os Optimize for size. -Os enables all -O2 optimizations that do not
typically increase code size. It also performs further
optimizations designed to reduce code size.
-Os disables the following optimization flags: -falign-functions-falign-jumps-falign-loops-falign-labels-freorder-blocks-freorder-blocks-and-partition-fprefetch-loop-arrays-ftree-vect-loop-version-Ofast
Disregard strict standards compliance. -Ofast enables all -O3
optimizations. It also enables optimizations that are not valid
for all standard compliant programs. It turns on -ffast-math and
the Fortran-specific -fno-protect-parens and -fstack-arrays.
If you use multiple -O options, with or without level numbers, the
last such option is the one that is effective.
Options of the form -fflag specify machine-independent flags. Most
flags have both positive and negative forms; the negative form of -ffoo
would be -fno-foo. In the table below, only one of the forms is
listed---the one you typically will use. You can figure out the other
form by either removing no- or adding it.
The following options control specific optimizations. They are either
activated by -O options or are related to ones that are. You can use
the following flags in the rare cases when "fine-tuning" of
optimizations to be performed is desired.
-fno-default-inline
Do not make member functions inline by default merely because they
are defined inside the class scope (C++ only). Otherwise, when you
specify -O, member functions defined inside class scope are
compiled inline by default; i.e., you don't need to add inline in
front of the member function name.
-fno-defer-pop
Always pop the arguments to each function call as soon as that
function returns. For machines that must pop arguments after a
function call, the compiler normally lets arguments accumulate on
the stack for several function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
-fforward-propagate
Perform a forward propagation pass on RTL. The pass tries to
combine two instructions and checks if the result can be
simplified. If loop unrolling is active, two passes are performed
and the second is scheduled after loop unrolling.
This option is enabled by default at optimization levels -O, -O2,
-O3, -Os.
-ffp-contract=style-ffp-contract=off disables floating-point expression contraction.
-ffp-contract=fast enables floating-point expression contraction
such as forming of fused multiply-add operations if the target has
native support for them. -ffp-contract=on enables floating-point
expression contraction if allowed by the language standard. This
is currently not implemented and treated equal to
-ffp-contract=off.
The default is -ffp-contract=fast.
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions that don't
need one. This avoids the instructions to save, set up and restore
frame pointers; it also makes an extra register available in many
functions. Italsomakesdebuggingimpossibleonsomemachines.
On some machines, such as the VAX, this flag has no effect, because
the standard calling sequence automatically handles the frame
pointer and nothing is saved by pretending it doesn't exist. The
machine-description macro "FRAME_POINTER_REQUIRED" controls whether
a target machine supports this flag.
Starting with GCC version 4.6, the default setting (when not
optimizing for size) for 32-bit Linux x86 and 32-bit Darwin x86
targets has been changed to -fomit-frame-pointer. The default can
be reverted to -fno-omit-frame-pointer by configuring GCC with the
--enable-frame-pointer configure option.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-fno-inline
Do not expand any functions inline apart from those marked with the
"always_inline" attribute. This is the default when not
optimizing.
Single functions can be exempted from inlining by marking them with
the "noinline" attribute.
-finline-small-functions
Integrate functions into their callers when their body is smaller
than expected function call code (so overall size of program gets
smaller). The compiler heuristically decides which functions are
simple enough to be worth integrating in this way. This inlining
applies to all functions, even those not declared inline.
Enabled at level -O2.
-findirect-inlining
Inline also indirect calls that are discovered to be known at
compile time thanks to previous inlining. This option has any
effect only when inlining itself is turned on by the
-finline-functions or -finline-small-functions options.
Enabled at level -O2.
-finline-functions
Consider all functions for inlining, even if they are not declared
inline. The compiler heuristically decides which functions are
worth integrating in this way.
If all calls to a given function are integrated, and the function
is declared "static", then the function is normally not output as
assembler code in its own right.
Enabled at level -O3.
-finline-functions-called-once
Consider all "static" functions called once for inlining into their
caller even if they are not marked "inline". If a call to a given
function is integrated, then the function is not output as
assembler code in its own right.
Enabled at levels -O1, -O2, -O3 and -Os.
-fearly-inlining
Inline functions marked by "always_inline" and functions whose body
seems smaller than the function call overhead early before doing
-fprofile-generate instrumentation and real inlining pass. Doing
so makes profiling significantly cheaper and usually inlining
faster on programs having large chains of nested wrapper functions.
Enabled by default.
-fipa-sra
Perform interprocedural scalar replacement of aggregates, removal
of unused parameters and replacement of parameters passed by
reference by parameters passed by value.
Enabled at levels -O2, -O3 and -Os.
-finline-limit=n
By default, GCC limits the size of functions that can be inlined.
This flag allows coarse control of this limit. n is the size of
functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which
may be specified individually by using --paramname=value. The
-finline-limit=n option sets some of these parameters as follows:
max-inline-insns-single
is set to n/2.
max-inline-insns-auto
is set to n/2.
See below for a documentation of the individual parameters
controlling inlining and for the defaults of these parameters.
Note: there may be no value to -finline-limit that results in
default behavior.
Note: pseudo instruction represents, in this particular context, an
abstract measurement of function's size. In no way does it
represent a count of assembly instructions and as such its exact
meaning might change from one release to an another.
-fno-keep-inline-dllexport
This is a more fine-grained version of -fkeep-inline-functions,
which applies only to functions that are declared using the
"dllexport" attribute or declspec
-fkeep-inline-functions
In C, emit "static" functions that are declared "inline" into the
object file, even if the function has been inlined into all of its
callers. This switch does not affect functions using the "extern
inline" extension in GNU C90. In C++, emit any and all inline
functions into the object file.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't
turned on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the
compiler to check if the variable was referenced, regardless of
whether or not optimization is turned on, use the
-fno-keep-static-consts option.
-fmerge-constants
Attempt to merge identical constants (string constants and
floating-point constants) across compilation units.
This option is the default for optimized compilation if the
assembler and linker support it. Use -fno-merge-constants to
inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to
-fmerge-constants this considers e.g. even constant initialized
arrays or initialized constant variables with integral or floating-
point types. Languages like C or C++ require each variable,
including multiple instances of the same variable in recursive
calls, to have distinct locations, so using this option will result
in non-conforming behavior.
-fmodulo-sched
Perform swing modulo scheduling immediately before the first
scheduling pass. This pass looks at innermost loops and reorders
their instructions by overlapping different iterations.
-fmodulo-sched-allow-regmoves
Perform more aggressive SMS based modulo scheduling with register
moves allowed. By setting this flag certain anti-dependences edges
will be deleted which will trigger the generation of reg-moves
based on the life-range analysis. This option is effective only
with -fmodulo-sched enabled.
-fno-branch-count-reg
Do not use "decrement and branch" instructions on a count register,
but instead generate a sequence of instructions that decrement a
register, compare it against zero, then branch based upon the
result. This option is only meaningful on architectures that
support such instructions, which include x86, PowerPC, IA-64 and
S/390.
The default is -fbranch-count-reg.
-fno-function-cse
Do not put function addresses in registers; make each instruction
that calls a constant function contain the function's address
explicitly.
This option results in less efficient code, but some strange hacks
that alter the assembler output may be confused by the
optimizations performed when this option is not used.
The default is -ffunction-cse-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables
that are initialized to zero into BSS. This can save space in the
resulting code.
This option turns off this behavior because some programs
explicitly rely on variables going to the data section. E.g., so
that the resulting executable can find the beginning of that
section and/or make assumptions based on that.
The default is -fzero-initialized-in-bss.
-fmudflap-fmudflapth-fmudflapir
For front-ends that support it (C and C++), instrument all risky
pointer/array dereferencing operations, some standard library
string/heap functions, and some other associated constructs with
range/validity tests. Modules so instrumented should be immune to
buffer overflows, invalid heap use, and some other classes of C/C++
programming errors. The instrumentation relies on a separate
runtime library (libmudflap), which will be linked into a program
if -fmudflap is given at link time. Run-time behavior of the
instrumented program is controlled by the MUDFLAP_OPTIONS
environment variable. See "env MUDFLAP_OPTIONS=-help a.out" for
its options.
Use -fmudflapth instead of -fmudflap to compile and to link if your
program is multi-threaded. Use -fmudflapir, in addition to
-fmudflap or -fmudflapth, if instrumentation should ignore pointer
reads. This produces less instrumentation (and therefore faster
execution) and still provides some protection against outright
memory corrupting writes, but allows erroneously read data to
propagate within a program.
-fthread-jumps
Perform optimizations where we check to see if a jump branches to a
location where another comparison subsumed by the first is found.
If so, the first branch is redirected to either the destination of
the second branch or a point immediately following it, depending on
whether the condition is known to be true or false.
Enabled at levels -O2, -O3, -Os.
-fsplit-wide-types
When using a type that occupies multiple registers, such as "long
long" on a 32-bit system, split the registers apart and allocate
them independently. This normally generates better code for those
types, but may make debugging more difficult.
Enabled at levels -O, -O2, -O3, -Os.
-fcse-follow-jumps
In common subexpression elimination (CSE), scan through jump
instructions when the target of the jump is not reached by any
other path. For example, when CSE encounters an "if" statement
with an "else" clause, CSE will follow the jump when the condition
tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow
jumps that conditionally skip over blocks. When CSE encounters a
simple "if" statement with no else clause, -fcse-skip-blocks causes
CSE to follow the jump around the body of the "if".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations
has been performed.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This pass
also performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC
extension, you may get better run-time performance if you disable
the global common subexpression elimination pass by adding
-fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination
will attempt to move loads that are only killed by stores into
themselves. This allows a loop containing a load/store sequence to
be changed to a load outside the loop, and a copy/store within the
loop.
Enabled by default when gcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global
common subexpression elimination. This pass will attempt to move
stores out of loops. When used in conjunction with -fgcse-lm,
loops containing a load/store sequence can be changed to a load
before the loop and a store after the loop.
Not enabled at any optimization level.
-fgcse-las
When -fgcse-las is enabled, the global common subexpression
elimination pass eliminates redundant loads that come after stores
to the same memory location (both partial and full redundancies).
Not enabled at any optimization level.
-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load elimination
pass is performed after reload. The purpose of this pass is to
cleanup redundant spilling.
-funsafe-loop-optimizations
If given, the loop optimizer will assume that loop indices do not
overflow, and that the loops with nontrivial exit condition are not
infinite. This enables a wider range of loop optimizations even if
the loop optimizer itself cannot prove that these assumptions are
valid. Using -Wunsafe-loop-optimizations, the compiler will warn
you if it finds this kind of loop.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies
equivalent code and save code size. The resulting code may or may
not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
-fauto-inc-dec
Combine increments or decrements of addresses with memory accesses.
This pass is always skipped on architectures that do not have
instructions to support this. Enabled by default at -O and higher
on architectures that support this.
-fdce
Perform dead code elimination (DCE) on RTL. Enabled by default at
-O and higher.
-fdse
Perform dead store elimination (DSE) on RTL. Enabled by default at
-O and higher.
-fif-conversion
Attempt to transform conditional jumps into branch-less
equivalents. This include use of conditional moves, min, max, set
flags and abs instructions, and some tricks doable by standard
arithmetics. The use of conditional execution on chips where it is
available is controlled by "if-conversion2".
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion2
Use conditional execution (where available) to transform
conditional jumps into branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os.
-fdelete-null-pointer-checks
Assume that programs cannot safely dereference null pointers, and
that no code or data element resides there. This enables simple
constant folding optimizations at all optimization levels. In
addition, other optimization passes in GCC use this flag to control
global dataflow analyses that eliminate useless checks for null
pointers; these assume that if a pointer is checked after it has
already been dereferenced, it cannot be null.
Note however that in some environments this assumption is not true.
Use -fno-delete-null-pointer-checks to disable this optimization
for programs that depend on that behavior.
Some targets, especially embedded ones, disable this option at all
levels. Otherwise it is enabled at all levels: -O0, -O1, -O2, -O3,
-Os. Passes that use the information are enabled independently at
different optimization levels.
-fdevirtualize
Attempt to convert calls to virtual functions to direct calls.
This is done both within a procedure and interprocedurally as part
of indirect inlining ("-findirect-inlining") and interprocedural
constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively
expensive.
Enabled at levels -O2, -O3, -Os.
-free
Attempt to remove redundant extension instructions. This is
especially helpful for the x86-64 architecture which implicitly
zero-extends in 64-bit registers after writing to their lower
32-bit half.
Enabled for x86 at levels -O2, -O3.
-foptimize-register-move-fregmove
Attempt to reassign register numbers in move instructions and as
operands of other simple instructions in order to maximize the
amount of register tying. This is especially helpful on machines
with two-operand instructions.
Note -fregmove and -foptimize-register-move are the same
optimization.
Enabled at levels -O2, -O3, -Os.
-fira-algorithm=algorithm
Use the specified coloring algorithm for the integrated register
allocator. The algorithm argument can be priority, which specifies
Chow's priority coloring, or CB, which specifies Chaitin-Briggs
coloring. Chaitin-Briggs coloring is not implemented for all
architectures, but for those targets that do support it, it is the
default because it generates better code.
-fira-region=region
Use specified regions for the integrated register allocator. The
region argument should be one of the following:
all Use all loops as register allocation regions. This can give
the best results for machines with a small and/or irregular
register set.
mixed
Use all loops except for loops with small register pressure as
the regions. This value usually gives the best results in most
cases and for most architectures, and is enabled by default
when compiling with optimization for speed (-O, -O2, ...).
one Use all functions as a single region. This typically results
in the smallest code size, and is enabled by default for -Os or
-O0.
-fira-loop-pressure
Use IRA to evaluate register pressure in loops for decisions to
move loop invariants. This option usually results in generation of
faster and smaller code on machines with large register files (>=
32 registers), but it can slow the compiler down.
This option is enabled at level -O3 for some targets.
-fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used hard
registers living through a call. Each hard register gets a
separate stack slot, and as a result function stack frames are
larger.
-fno-ira-share-spill-slots
Disable sharing of stack slots allocated for pseudo-registers.
Each pseudo-register that does not get a hard register gets a
separate stack slot, and as a result function stack frames are
larger.
-fira-verbose=n
Control the verbosity of the dump file for the integrated register
allocator. The default value is 5. If the value n is greater or
equal to 10, the dump output is sent to stderr using the same
format as n minus 10.
-fdelayed-branch
If supported for the target machine, attempt to reorder
instructions to exploit instruction slots available after delayed
branch instructions.
Enabled at levels -O, -O2, -O3, -Os.
-fschedule-insns
If supported for the target machine, attempt to reorder
instructions to eliminate execution stalls due to required data
being unavailable. This helps machines that have slow floating
point or memory load instructions by allowing other instructions to
be issued until the result of the load or floating-point
instruction is required.
Enabled at levels -O2, -O3.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of
instruction scheduling after register allocation has been done.
This is especially useful on machines with a relatively small
number of registers and where memory load instructions take more
than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Don't schedule instructions across basic blocks. This is normally
enabled by default when scheduling before register allocation, i.e.
with -fschedule-insns or at -O2 or higher.
-fno-sched-spec
Don't allow speculative motion of non-load instructions. This is
normally enabled by default when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-pressure
Enable register pressure sensitive insn scheduling before the
register allocation. This only makes sense when scheduling before
register allocation is enabled, i.e. with -fschedule-insns or at
-O2 or higher. Usage of this option can improve the generated code
and decrease its size by preventing register pressure increase
above the number of available hard registers and as a consequence
register spills in the register allocation.
-fsched-spec-load
Allow speculative motion of some load instructions. This only
makes sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only
makes sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
-fsched-stalled-insns-fsched-stalled-insns=n
Define how many insns (if any) can be moved prematurely from the
queue of stalled insns into the ready list, during the second
scheduling pass. -fno-sched-stalled-insns means that no insns will
be moved prematurely, -fsched-stalled-insns=0 means there is no
limit on how many queued insns can be moved prematurely.
-fsched-stalled-insns without a value is equivalent to
-fsched-stalled-insns=1.
-fsched-stalled-insns-dep-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) will be examined for a
dependency on a stalled insn that is candidate for premature
removal from the queue of stalled insns. This has an effect only
during the second scheduling pass, and only if
-fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is
equivalent to -fsched-stalled-insns-dep=0.
-fsched-stalled-insns-dep without a value is equivalent to
-fsched-stalled-insns-dep=1.
-fsched2-use-superblocks
When scheduling after register allocation, do use superblock
scheduling algorithm. Superblock scheduling allows motion across
basic block boundaries resulting on faster schedules. This option
is experimental, as not all machine descriptions used by GCC model
the CPU closely enough to avoid unreliable results from the
algorithm.
This only makes sense when scheduling after register allocation,
i.e. with -fschedule-insns2 or at -O2 or higher.
-fsched-group-heuristic
Enable the group heuristic in the scheduler. This heuristic favors
the instruction that belongs to a schedule group. This is enabled
by default when scheduling is enabled, i.e. with -fschedule-insns
or -fschedule-insns2 or at -O2 or higher.
-fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler. This
heuristic favors instructions on the critical path. This is
enabled by default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the scheduler.
This heuristic favors speculative instructions with greater
dependency weakness. This is enabled by default when scheduling is
enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2
or higher.
-fsched-rank-heuristic
Enable the rank heuristic in the scheduler. This heuristic favors
the instruction belonging to a basic block with greater size or
frequency. This is enabled by default when scheduling is enabled,
i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or
higher.
-fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler. This
heuristic favors the instruction that is less dependent on the last
instruction scheduled. This is enabled by default when scheduling
is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at
-O2 or higher.
-fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler. This
heuristic favors the instruction that has more instructions
depending on it. This is enabled by default when scheduling is
enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2
or higher.
-freschedule-modulo-scheduled-loops
The modulo scheduling comes before the traditional scheduling, if a
loop was modulo scheduled we may want to prevent the later
scheduling passes from changing its schedule, we use this option to
control that.
-fselective-scheduling
Schedule instructions using selective scheduling algorithm.
Selective scheduling runs instead of the first scheduler pass.
-fselective-scheduling2
Schedule instructions using selective scheduling algorithm.
Selective scheduling runs instead of the second scheduler pass.
-fsel-sched-pipelining
Enable software pipelining of innermost loops during selective
scheduling. This option has no effect until one of
-fselective-scheduling or -fselective-scheduling2 is turned on.
-fsel-sched-pipelining-outer-loops
When pipelining loops during selective scheduling, also pipeline
outer loops. This option has no effect until
-fsel-sched-pipelining is turned on.
-fshrink-wrap
Emit function prologues only before parts of the function that need
it, rather than at the top of the function. This flag is enabled
by default at -O and higher.
-fcaller-saves
Enable values to be allocated in registers that will be clobbered
by function calls, by emitting extra instructions to save and
restore the registers around such calls. Such allocation is done
only when it seems to result in better code than would otherwise be
produced.
This option is always enabled by default on certain machines,
usually those which have no call-preserved registers to use
instead.
Enabled at levels -O2, -O3, -Os.
-fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack memory
references and then tries to find ways to combine them.
Enabled by default at -O1 and higher.
-fconserve-stack
Attempt to minimize stack usage. The compiler will attempt to use
less stack space, even if that makes the program slower. This
option implies setting the large-stack-frame parameter to 100 and
the large-stack-frame-growth parameter to 400.
-ftree-reassoc
Perform reassociation on trees. This flag is enabled by default at
-O and higher.
-ftree-pre
Perform partial redundancy elimination (PRE) on trees. This flag
is enabled by default at -O2 and -O3.
-ftree-forwprop
Perform forward propagation on trees. This flag is enabled by
default at -O and higher.
-ftree-fre
Perform full redundancy elimination (FRE) on trees. The difference
between FRE and PRE is that FRE only considers expressions that are
computed on all paths leading to the redundant computation. This
analysis is faster than PRE, though it exposes fewer redundancies.
This flag is enabled by default at -O and higher.
-ftree-phiprop
Perform hoisting of loads from conditional pointers on trees. This
pass is enabled by default at -O and higher.
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates
unnecessary copy operations. This flag is enabled by default at -O
and higher.
-fipa-pure-const
Discover which functions are pure or constant. Enabled by default
at -O and higher.
-fipa-reference
Discover which static variables do not escape cannot escape the
compilation unit. Enabled by default at -O and higher.
-fipa-pta
Perform interprocedural pointer analysis and interprocedural
modification and reference analysis. This option can cause
excessive memory and compile-time usage on large compilation units.
It is not enabled by default at any optimization level.
-fipa-profile
Perform interprocedural profile propagation. The functions called
only from cold functions are marked as cold. Also functions
executed once (such as "cold", "noreturn", static constructors or
destructors) are identified. Cold functions and loop less parts of
functions executed once are then optimized for size. Enabled by
default at -O and higher.
-fipa-cp
Perform interprocedural constant propagation. This optimization
analyzes the program to determine when values passed to functions
are constants and then optimizes accordingly. This optimization
can substantially increase performance if the application has
constants passed to functions. This flag is enabled by default at
-O2, -Os and -O3.
-fipa-cp-clone
Perform function cloning to make interprocedural constant
propagation stronger. When enabled, interprocedural constant
propagation will perform function cloning when externally visible
function can be called with constant arguments. Because this
optimization can create multiple copies of functions, it may
significantly increase code size (see --paramipcp-unit-growth=value). This flag is enabled by default at -O3.
-fipa-matrix-reorg
Perform matrix flattening and transposing. Matrix flattening tries
to replace an m-dimensional matrix with its equivalent
n-dimensional matrix, where n < m. This reduces the level of
indirection needed for accessing the elements of the matrix. The
second optimization is matrix transposing, which attempts to change
the order of the matrix's dimensions in order to improve cache
locality. Both optimizations need the -fwhole-program flag.
Transposing is enabled only if profiling information is available.
-ftree-sink
Perform forward store motion on trees. This flag is enabled by
default at -O and higher.
-ftree-bit-ccp
Perform sparse conditional bit constant propagation on trees and
propagate pointer alignment information. This pass only operates
on local scalar variables and is enabled by default at -O and
higher. It requires that -ftree-ccp is enabled.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees.
This pass only operates on local scalar variables and is enabled by
default at -O and higher.
-ftree-switch-conversion
Perform conversion of simple initializations in a switch to
initializations from a scalar array. This flag is enabled by
default at -O2 and higher.
-ftree-tail-merge
Look for identical code sequences. When found, replace one with a
jump to the other. This optimization is known as tail merging or
cross jumping. This flag is enabled by default at -O2 and higher.
The compilation time in this pass can be limited using max-tail-merge-comparisons parameter and max-tail-merge-iterations
parameter.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled
by default at -O and higher.
-ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for calls to
builtin functions that may set "errno" but are otherwise side-
effect free. This flag is enabled by default at -O2 and higher if
-Os is not also specified.
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy
propagation, redundancy elimination, range propagation and
expression simplification) based on a dominator tree traversal.
This also performs jump threading (to reduce jumps to jumps). This
flag is enabled by default at -O and higher.
-ftree-dse
Perform dead store elimination (DSE) on trees. A dead store is a
store into a memory location that is later overwritten by another
store without any intervening loads. In this case the earlier
store can be deleted. This flag is enabled by default at -O and
higher.
-ftree-ch
Perform loop header copying on trees. This is beneficial since it
increases effectiveness of code motion optimizations. It also
saves one jump. This flag is enabled by default at -O and higher.
It is not enabled for -Os, since it usually increases code size.
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by
default at -O and higher.
-ftree-loop-linear
Perform loop interchange transformations on tree. Same as
-floop-interchange. To use this code transformation, GCC has to be
configured with --with-ppl and --with-cloog to enable the Graphite
loop transformation infrastructure.
-floop-interchange
Perform loop interchange transformations on loops. Interchanging
two nested loops switches the inner and outer loops. For example,
given a loop like:
DO J = 1, M
DO I = 1, N
A(J, I) = A(J, I) * C
ENDDO
ENDDO
loop interchange will transform the loop as if the user had
written:
DO I = 1, N
DO J = 1, M
A(J, I) = A(J, I) * C
ENDDO
ENDDO
which can be beneficial when "N" is larger than the caches, because
in Fortran, the elements of an array are stored in memory
contiguously by column, and the original loop iterates over rows,
potentially creating at each access a cache miss. This
optimization applies to all the languages supported by GCC and is
not limited to Fortran. To use this code transformation, GCC has
to be configured with --with-ppl and --with-cloog to enable the
Graphite loop transformation infrastructure.
-floop-strip-mine
Perform loop strip mining transformations on loops. Strip mining
splits a loop into two nested loops. The outer loop has strides
equal to the strip size and the inner loop has strides of the
original loop within a strip. The strip length can be changed
using the loop-block-tile-size parameter. For example, given a
loop like:
DO I = 1, N
A(I) = A(I) + C
ENDDO
loop strip mining will transform the loop as if the user had
written:
DO II = 1, N, 51
DO I = II, min (II + 50, N)
A(I) = A(I) + C
ENDDO
ENDDO
This optimization applies to all the languages supported by GCC and
is not limited to Fortran. To use this code transformation, GCC
has to be configured with --with-ppl and --with-cloog to enable the
Graphite loop transformation infrastructure.
-floop-block
Perform loop blocking transformations on loops. Blocking strip
mines each loop in the loop nest such that the memory accesses of
the element loops fit inside caches. The strip length can be
changed using the loop-block-tile-size parameter. For example,
given a loop like:
DO I = 1, N
DO J = 1, M
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
loop blocking will transform the loop as if the user had written:
DO II = 1, N, 51
DO JJ = 1, M, 51
DO I = II, min (II + 50, N)
DO J = JJ, min (JJ + 50, M)
A(J, I) = B(I) + C(J)
ENDDO
ENDDO
ENDDO
ENDDO
which can be beneficial when "M" is larger than the caches, because
the innermost loop will iterate over a smaller amount of data which
can be kept in the caches. This optimization applies to all the
languages supported by GCC and is not limited to Fortran. To use
this code transformation, GCC has to be configured with --with-ppl
and --with-cloog to enable the Graphite loop transformation
infrastructure.
-fgraphite-identity
Enable the identity transformation for graphite. For every SCoP we
generate the polyhedral representation and transform it back to
gimple. Using -fgraphite-identity we can check the costs or
benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some
minimal optimizations are also performed by the code generator
CLooG, like index splitting and dead code elimination in loops.
-floop-flatten
Removes the loop nesting structure: transforms the loop nest into a
single loop. This transformation can be useful as an enablement
transform for vectorization and parallelization. This feature is
experimental. To use this code transformation, GCC has to be
configured with --with-ppl and --with-cloog to enable the Graphite
loop transformation infrastructure.
-floop-parallelize-all
Use the Graphite data dependence analysis to identify loops that
can be parallelized. Parallelize all the loops that can be
analyzed to not contain loop carried dependences without checking
that it is profitable to parallelize the loops.
-fcheck-data-deps
Compare the results of several data dependence analyzers. This
option is used for debugging the data dependence analyzers.
-ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost loops to
branch-less equivalents. The intent is to remove control-flow from
the innermost loops in order to improve the ability of the
vectorization pass to handle these loops. This is enabled by
default if vectorization is enabled.
-ftree-loop-if-convert-stores
Attempt to also if-convert conditional jumps containing memory
writes. This transformation can be unsafe for multi-threaded
programs as it transforms conditional memory writes into
unconditional memory writes. For example,
for (i = 0; i < N; i++)
if (cond)
A[i] = expr;
would be transformed to
for (i = 0; i < N; i++)
A[i] = cond ? expr : A[i];
potentially producing data races.
-ftree-loop-distribution
Perform loop distribution. This flag can improve cache performance
on big loop bodies and allow further loop optimizations, like
parallelization or vectorization, to take place. For example, the
loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
is transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
-ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code generated
with calls to a library. This flag is enabled by default at -O3.
This pass distributes the initialization loops and generates a call
to memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and the initialization loop is transformed into a call to memset
zero.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only
invariants that would be hard to handle at RTL level (function
calls, operations that expand to nontrivial sequences of insns).
With -funswitch-loops it also moves operands of conditions that are
invariant out of the loop, so that we can use just trivial
invariantness analysis in loop unswitching. The pass also includes
store motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in loops for
which determining number of iterations requires complicated
analysis. Later optimizations then may determine the number
easily. Useful especially in connection with unrolling.
-fivopts
Perform induction variable optimizations (strength reduction,
induction variable merging and induction variable elimination) on
trees.
-ftree-parallelize-loops=n
Parallelize loops, i.e., split their iteration space to run in n
threads. This is only possible for loops whose iterations are
independent and can be arbitrarily reordered. The optimization is
only profitable on multiprocessor machines, for loops that are CPU-
intensive, rather than constrained e.g. by memory bandwidth. This
option implies -pthread, and thus is only supported on targets that
have support for -pthread.
-ftree-pta
Perform function-local points-to analysis on trees. This flag is
enabled by default at -O and higher.
-ftree-sra
Perform scalar replacement of aggregates. This pass replaces
structure references with scalars to prevent committing structures
to memory too early. This flag is enabled by default at -O and
higher.
-ftree-copyrename
Perform copy renaming on trees. This pass attempts to rename
compiler temporaries to other variables at copy locations, usually
resulting in variable names which more closely resemble the
original variables. This flag is enabled by default at -O and
higher.
-ftree-ter
Perform temporary expression replacement during the SSA->normal
phase. Single use/single def temporaries are replaced at their use
location with their defining expression. This results in non-
GIMPLE code, but gives the expanders much more complex trees to
work on resulting in better RTL generation. This is enabled by
default at -O and higher.
-ftree-vectorize
Perform loop vectorization on trees. This flag is enabled by
default at -O3.
-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is enabled by
default at -O3 and when -ftree-vectorize is enabled.
-ftree-vect-loop-version
Perform loop versioning when doing loop vectorization on trees.
When a loop appears to be vectorizable except that data alignment
or data dependence cannot be determined at compile time, then
vectorized and non-vectorized versions of the loop are generated
along with run-time checks for alignment or dependence to control
which version is executed. This option is enabled by default
except at level -Os where it is disabled.
-fvect-cost-model
Enable cost model for vectorization.
-ftree-vrp
Perform Value Range Propagation on trees. This is similar to the
constant propagation pass, but instead of values, ranges of values
are propagated. This allows the optimizers to remove unnecessary
range checks like array bound checks and null pointer checks. This
is enabled by default at -O2 and higher. Null pointer check
elimination is only done if -fdelete-null-pointer-checks is
enabled.
-ftracer
Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function allowing
other optimizations to do better job.
-funroll-loops
Unroll loops whose number of iterations can be determined at
compile time or upon entry to the loop. -funroll-loops implies
-frerun-cse-after-loop. This option makes code larger, and may or
may not make it run faster.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain
when the loop is entered. This usually makes programs run more
slowly. -funroll-all-loops implies the same options as
-funroll-loops,
-fsplit-ivs-in-unroller
Enables expressing of values of induction variables in later
iterations of the unrolled loop using the value in the first
iteration. This breaks long dependency chains, thus improving
efficiency of the scheduling passes.
Combination of -fweb and CSE is often sufficient to obtain the same
effect. However in cases the loop body is more complicated than a
single basic block, this is not reliable. It also does not work at
all on some of the architectures due to restrictions in the CSE
pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler will create multiple copies of some
local variables when unrolling a loop which can result in superior
code.
-fpartial-inlining
Inline parts of functions. This option has any effect only when
inlining itself is turned on by the -finline-functions or
-finline-small-functions options.
Enabled at level -O2.
-fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing
computations (especially memory loads and stores) performed in
previous iterations of loops.
This option is enabled at level -O3.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to
prefetch memory to improve the performance of loops that access
large arrays.
This option may generate better or worse code; results are highly
dependent on the structure of loops within the source code.
Disabled at level -Os.
-fno-peephole-fno-peephole2
Disable any machine-specific peephole optimizations. The
difference between -fno-peephole and -fno-peephole2 is in how they
are implemented in the compiler; some targets use one, some use the
other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels
-O2, -O3, -Os.
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC will use heuristics to guess branch probabilities if they are
not provided by profiling feedback (-fprofile-arcs). These
heuristics are based on the control flow graph. If some branch
probabilities are specified by __builtin_expect, then the
heuristics will be used to guess branch probabilities for the rest
of the control flow graph, taking the __builtin_expect info into
account. The interactions between the heuristics and
__builtin_expect can be complex, and in some cases, it may be
useful to disable the heuristics so that the effects of
__builtin_expect are easier to understand.
The default is -fguess-branch-probability at levels -O, -O2, -O3,
-Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce
number of taken branches and improve code locality.
Enabled at levels -O2, -O3.
-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function, in
order to reduce number of taken branches, partitions hot and cold
basic blocks into separate sections of the assembly and .o files,
to improve paging and cache locality performance.
This optimization is automatically turned off in the presence of
exception handling, for linkonce sections, for functions with a
user-defined section attribute and on any architecture that does
not support named sections.
-freorder-functions
Reorder functions in the object file in order to improve code
locality. This is implemented by using special subsections
".text.hot" for most frequently executed functions and
".text.unlikely" for unlikely executed functions. Reordering is
done by the linker so object file format must support named
sections and linker must place them in a reasonable way.
Also profile feedback must be available in to make this option
effective. See -fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules
applicable to the language being compiled. For C (and C++), this
activates optimizations based on the type of expressions. In
particular, an object of one type is assumed never to reside at the
same address as an object of a different type, unless the types are
almost the same. For example, an "unsigned int" can alias an
"int", but not a "void*" or a "double". A character type may alias
any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one
most recently written to (called "type-punning") is common. Even
with -fstrict-aliasing, type-punning is allowed, provided the
memory is accessed through the union type. So, the code above will
work as expected. However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the resulting
pointer and dereferencing the result has undefined behavior, even
if the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
-fstrict-overflow
Allow the compiler to assume strict signed overflow rules,
depending on the language being compiled. For C (and C++) this
means that overflow when doing arithmetic with signed numbers is
undefined, which means that the compiler may assume that it will
not happen. This permits various optimizations. For example, the
compiler will assume that an expression like "i + 10 > i" will
always be true for signed "i". This assumption is only valid if
signed overflow is undefined, as the expression is false if "i +
10" overflows when using twos complement arithmetic. When this
option is in effect any attempt to determine whether an operation
on signed numbers will overflow must be written carefully to not
actually involve overflow.
This option also allows the compiler to assume strict pointer
semantics: given a pointer to an object, if adding an offset to
that pointer does not produce a pointer to the same object, the
addition is undefined. This permits the compiler to conclude that
"p + u > p" is always true for a pointer "p" and unsigned integer
"u". This assumption is only valid because pointer wraparound is
undefined, as the expression is false if "p + u" overflows using
twos complement arithmetic.
See also the -fwrapv option. Using -fwrapv means that integer
signed overflow is fully defined: it wraps. When -fwrapv is used,
there is no difference between -fstrict-overflow and
-fno-strict-overflow for integers. With -fwrapv certain types of
overflow are permitted. For example, if the compiler gets an
overflow when doing arithmetic on constants, the overflowed value
can still be used with -fwrapv, but not otherwise.
The -fstrict-overflow option is enabled at levels -O2, -O3, -Os.
-falign-functions-falign-functions=n
Align the start of functions to the next power-of-two greater than
n, skipping up to n bytes. For instance, -falign-functions=32
aligns functions to the next 32-byte boundary, but
-falign-functions=24 would align to the next 32-byte boundary only
if this can be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent and
mean that functions will not be aligned.
Some assemblers only support this flag when n is a power of two; in
that case, it is rounded up.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-labels-falign-labels=n
Align all branch targets to a power-of-two boundary, skipping up to
n bytes like -falign-functions. This option can easily make code
slower, because it must insert dummy operations for when the branch
target is reached in the usual flow of the code.
-fno-align-labels and -falign-labels=1 are equivalent and mean that
labels will not be aligned.
If -falign-loops or -falign-jumps are applicable and are greater
than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default
which is very likely to be 1, meaning no alignment.
Enabled at levels -O2, -O3.
-falign-loops-falign-loops=n
Align loops to a power-of-two boundary, skipping up to n bytes like
-falign-functions. The hope is that the loop will be executed many
times, which will make up for any execution of the dummy
operations.
-fno-align-loops and -falign-loops=1 are equivalent and mean that
loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps-falign-jumps=n
Align branch targets to a power-of-two boundary, for branch targets
where the targets can only be reached by jumping, skipping up to n
bytes like -falign-functions. In this case, no dummy operations
need be executed.
-fno-align-jumps and -falign-jumps=1 are equivalent and mean that
loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-funit-at-a-time
This option is left for compatibility reasons. -funit-at-a-time has
no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder
and -fno-section-anchors.
Enabled by default.
-fno-toplevel-reorder
Do not reorder top-level functions, variables, and "asm"
statements. Output them in the same order that they appear in the
input file. When this option is used, unreferenced static
variables will not be removed. This option is intended to support
existing code that relies on a particular ordering. For new code,
it is better to use attributes.
Enabled at level -O0. When disabled explicitly, it also implies
-fno-section-anchors, which is otherwise enabled at -O0 on some
targets.
-fweb
Constructs webs as commonly used for register allocation purposes
and assign each web individual pseudo register. This allows the
register allocation pass to operate on pseudos directly, but also
strengthens several other optimization passes, such as CSE, loop
optimizer and trivial dead code remover. It can, however, make
debugging impossible, since variables will no longer stay in a
"home register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents the whole
program being compiled. All public functions and variables with
the exception of "main" and those merged by attribute
"externally_visible" become static functions and in effect are
optimized more aggressively by interprocedural optimizers. If gold
is used as the linker plugin, "externally_visible" attributes are
automatically added to functions (not variable yet due to a current
gold issue) that are accessed outside of LTO objects according to
resolution file produced by gold. For other linkers that cannot
generate resolution file, explicit "externally_visible" attributes
are still necessary. While this option is equivalent to proper use
of the "static" keyword for programs consisting of a single file,
in combination with option -flto this flag can be used to compile
many smaller scale programs since the functions and variables
become local for the whole combined compilation unit, not for the
single source file itself.
This option implies -fwhole-file for Fortran programs.
-flto[=n]
This option runs the standard link-time optimizer. When invoked
with source code, it generates GIMPLE (one of GCC's internal
representations) and writes it to special ELF sections in the
object file. When the object files are linked together, all the
function bodies are read from these ELF sections and instantiated
as if they had been part of the same translation unit.
To use the link-time optimizer, -flto needs to be specified at
compile time and during the final link. For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation of
GIMPLE into special ELF sections inside foo.o and bar.o. The final
invocation reads the GIMPLE bytecode from foo.o and bar.o, merges
the two files into a single internal image, and compiles the result
as usual. Since both foo.o and bar.o are merged into a single
image, this causes all the interprocedural analyses and
optimizations in GCC to work across the two files as if they were a
single one. This means, for example, that the inliner is able to
inline functions in bar.o into functions in foo.o and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges them
together into a single GIMPLE representation and optimizes them as
usual to produce myprog.
The only important thing to keep in mind is that to enable link-
time optimizations the -flto flag needs to be passed to both the
compile and the link commands.
To make whole program optimization effective, it is necessary to
make certain whole program assumptions. The compiler needs to know
what functions and variables can be accessed by libraries and
runtime outside of the link-time optimized unit. When supported by
the linker, the linker plugin (see -fuse-linker-plugin) passes
information to the compiler about used and externally visible
symbols. When the linker plugin is not available, -fwhole-program
should be used to allow the compiler to make these assumptions,
which leads to more aggressive optimization decisions.
Note that when a file is compiled with -flto, the generated object
file is larger than a regular object file because it contains
GIMPLE bytecodes and the usual final code. This means that object
files with LTO information can be linked as normal object files; if
-flto is not passed to the linker, no interprocedural optimizations
are applied.
Additionally, the optimization flags used to compile individual
files are not necessarily related to those used at link time. For
instance,
gcc -c -O0 -flto foo.c
gcc -c -O0 -flto bar.c
gcc -o myprog -flto -O3 foo.o bar.o
This produces individual object files with unoptimized assembler
code, but the resulting binary myprog is optimized at -O3. If,
instead, the final binary is generated without -flto, then myprog
is not optimized.
When producing the final binary with -flto, GCC only applies link-
time optimizations to those files that contain bytecode.
Therefore, you can mix and match object files and libraries with
GIMPLE bytecodes and final object code. GCC automatically selects
which files to optimize in LTO mode and which files to link without
further processing.
There are some code generation flags preserved by GCC when
generating bytecodes, as they need to be used during the final link
stage. Currently, the following options are saved into the GIMPLE
bytecode files: -fPIC, -fcommon and all the -m target flags.
At link time, these options are read in and reapplied. Note that
the current implementation makes no attempt to recognize
conflicting values for these options. If different files have
conflicting option values (e.g., one file is compiled with -fPIC
and another isn't), the compiler simply uses the last value read
from the bytecode files. It is recommended, then, that you compile
all the files participating in the same link with the same options.
If LTO encounters objects with C linkage declared with incompatible
types in separate translation units to be linked together
(undefined behavior according to ISO C99 6.2.7), a non-fatal
diagnostic may be issued. The behavior is still undefined at run
time.
Another feature of LTO is that it is possible to apply
interprocedural optimizations on files written in different
languages. This requires support in the language front end.
Currently, the C, C++ and Fortran front ends are capable of
emitting GIMPLE bytecodes, so something like this should work:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++ runtime
libraries and -lgfortran is added to get the Fortran runtime
libraries. In general, when mixing languages in LTO mode, you
should use the same link command options as when mixing languages
in a regular (non-LTO) compilation; all you need to add is -flto to
all the compile and link commands.
If object files containing GIMPLE bytecode are stored in a library
archive, say libfoo.a, it is possible to extract and use them in an
LTO link if you are using a linker with plugin support. To enable
this feature, use the flag -fuse-linker-plugin at link time:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed
GIMPLE files from libfoo.a and passes them on to the running GCC to
make them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not
enable the linker plugin, then the objects inside libfoo.a are
extracted and linked as usual, but they do not participate in the
LTO optimization process.
Link-time optimizations do not require the presence of the whole
program to operate. If the program does not require any symbols to
be exported, it is possible to combine -flto and -fwhole-program to
allow the interprocedural optimizers to use more aggressive
assumptions which may lead to improved optimization opportunities.
Use of -fwhole-program is not needed when linker plugin is active
(see -fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate
bytecode that is portable between different types of hosts. The
bytecode files are versioned and there is a strict version check,
so bytecode files generated in one version of GCC will not work
with an older/newer version of GCC.
Link-time optimization does not work well with generation of
debugging information. Combining -flto with -g is currently
experimental and expected to produce wrong results.
If you specify the optional n, the optimization and code generation
done at link time is executed in parallel using n parallel jobs by
utilizing an installed make program. The environment variable MAKE
may be used to override the program used. The default value for n
is 1.
You can also specify -flto=jobserver to use GNU make's job server
mode to determine the number of parallel jobs. This is useful when
the Makefile calling GCC is already executing in parallel. You
must prepend a * to the command recipe in the parent Makefile for
this to work. This option likely only works if MAKE is GNU make.
This option is disabled by default
-flto-partition=alg
Specify the partitioning algorithm used by the link-time optimizer.
The value is either "1to1" to specify a partitioning mirroring the
original source files or "balanced" to specify partitioning into
equally sized chunks (whenever possible). Specifying "none" as an
algorithm disables partitioning and streaming completely. The
default value is "balanced".
-flto-compression-level=n
This option specifies the level of compression used for
intermediate language written to LTO object files, and is only
meaningful in conjunction with LTO mode (-flto). Valid values are
0 (no compression) to 9 (maximum compression). Values outside this
range are clamped to either 0 or 9. If the option is not given, a
default balanced compression setting is used.
-flto-report
Prints a report with internal details on the workings of the link-
time optimizer. The contents of this report vary from version to
version. It is meant to be useful to GCC developers when
processing object files in LTO mode (via -flto).
Disabled by default.
-fuse-linker-plugin
Enables the use of a linker plugin during link-time optimization.
This option relies on plugin support in the linker, which is
available in gold or in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE
bytecode out of library archives. This improves the quality of
optimization by exposing more code to the link-time optimizer.
This information specifies what symbols can be accessed externally
(by non-LTO object or during dynamic linking). Resulting code
quality improvements on binaries (and shared libraries that use
hidden visibility) are similar to "-fwhole-program". See -flto for
a description of the effect of this flag and how to use it.
This option is enabled by default when LTO support in GCC is
enabled and GCC was configured for use with a linker supporting
plugins (GNU ld 2.21 or newer or gold).
-ffat-lto-objects
Fat LTO objects are object files that contain both the intermediate
language and the object code. This makes them usable for both LTO
linking and normal linking. This option is effective only when
compiling with -flto and is ignored at link time.
-fno-fat-lto-objects improves compilation time over plain LTO, but
requires the complete toolchain to be aware of LTO. It requires a
linker with linker plugin support for basic functionality.
Additionally, nm, ar and ranlib need to support linker plugins to
allow a full-featured build environment (capable of building static
libraries etc).
The default is -ffat-lto-objects but this default is intended to
change in future releases when linker plugin enabled environments
become more common.
-fcompare-elim
After register allocation and post-register allocation instruction
splitting, identify arithmetic instructions that compute processor
flags similar to a comparison operation based on that arithmetic.
If possible, eliminate the explicit comparison operation.
This pass only applies to certain targets that cannot explicitly
represent the comparison operation before register allocation is
complete.
Enabled at levels -O, -O2, -O3, -Os.
-fcprop-registers
After register allocation and post-register allocation instruction
splitting, we perform a copy-propagation pass to try to reduce
scheduling dependencies and occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
-fprofile-correction
Profiles collected using an instrumented binary for multi-threaded
programs may be inconsistent due to missed counter updates. When
this option is specified, GCC will use heuristics to correct or
smooth out such inconsistencies. By default, GCC will emit an error
message when an inconsistent profile is detected.
-fprofile-dir=path
Set the directory to search for the profile data files in to path.
This option affects only the profile data generated by
-fprofile-generate, -ftest-coverage, -fprofile-arcs and used by
-fprofile-use and -fbranch-probabilities and its related options.
Both absolute and relative paths can be used. By default, GCC will
use the current directory as path, thus the profile data file will
appear in the same directory as the object file.
-fprofile-generate-fprofile-generate=path
Enable options usually used for instrumenting application to
produce profile useful for later recompilation with profile
feedback based optimization. You must use -fprofile-generate both
when compiling and when linking your program.
The following options are enabled: "-fprofile-arcs",
"-fprofile-values", "-fvpt".
If path is specified, GCC will look at the path to find the profile
feedback data files. See -fprofile-dir.
-fprofile-use-fprofile-use=path
Enable profile feedback directed optimizations, and optimizations
generally profitable only with profile feedback available.
The following options are enabled: "-fbranch-probabilities",
"-fvpt", "-funroll-loops", "-fpeel-loops", "-ftracer"
By default, GCC emits an error message if the feedback profiles do
not match the source code. This error can be turned into a warning
by using -Wcoverage-mismatch. Note this may result in poorly
optimized code.
If path is specified, GCC will look at the path to find the profile
feedback data files. See -fprofile-dir.
The following options control compiler behavior regarding floating-
point arithmetic. These options trade off between speed and
correctness. All must be specifically enabled.
-ffloat-store
Do not store floating-point variables in registers, and inhibit
other options that might change whether a floating-point value is
taken from a register or memory.
This option prevents undesirable excess precision on machines such
as the 68000 where the floating registers (of the 68881) keep more
precision than a "double" is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does
only good, but a few programs rely on the precise definition of
IEEE floating point. Use -ffloat-store for such programs, after
modifying them to store all pertinent intermediate computations
into variables.
-fexcess-precision=style
This option allows further control over excess precision on
machines where floating-point registers have more precision than
the IEEE "float" and "double" types and the processor does not
support operations rounding to those types. By default,
-fexcess-precision=fast is in effect; this means that operations
are carried out in the precision of the registers and that it is
unpredictable when rounding to the types specified in the source
code takes place. When compiling C, if -fexcess-precision=standard
is specified then excess precision will follow the rules specified
in ISO C99; in particular, both casts and assignments cause values
to be rounded to their semantic types (whereas -ffloat-store only
affects assignments). This option is enabled by default for C if a
strict conformance option such as -std=c99 is used.
-fexcess-precision=standard is not implemented for languages other
than C, and has no effect if -funsafe-math-optimizations or
-ffast-math is specified. On the x86, it also has no effect if
-mfpmath=sse or -mfpmath=sse+387 is specified; in the former case,
IEEE semantics apply without excess precision, and in the latter,
rounding is unpredictable.
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations,
-ffinite-math-only, -fno-rounding-math, -fno-signaling-nans and
-fcx-limited-range.
This option causes the preprocessor macro "__FAST_MATH__" to be
defined.
This option is not turned on by any -O option besides -Ofast since
it can result in incorrect output for programs that depend on an
exact implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications.
-fno-math-errno
Do not set ERRNO after calling math functions that are executed
with a single instruction, e.g., sqrt. A program that relies on
IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.
This option is not turned on by any -O option since it can result
in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is
therefore no reason for the compiler to consider the possibility
that it might, and -fno-math-errno is the default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume
that arguments and results are valid and (b) may violate IEEE or
ANSI standards. When used at link-time, it may include libraries
or startup files that change the default FPU control word or other
similar optimizations.
This option is not turned on by any -O option since it can result
in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications. Enables
-fno-signed-zeros, -fno-trapping-math, -fassociative-math and
-freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math
Allow re-association of operands in series of floating-point
operations. This violates the ISO C and C++ language standard by
possibly changing computation result. NOTE: re-ordering may change
the sign of zero as well as ignore NaNs and inhibit or create
underflow or overflow (and thus cannot be used on code that relies
on rounding behavior like "(x + 2**52) - 2**52". May also reorder
floating-point comparisons and thus may not be used when ordered
comparisons are required. This option requires that both
-fno-signed-zeros and -fno-trapping-math be in effect. Moreover,
it doesn't make much sense with -frounding-math. For Fortran the
option is automatically enabled when both -fno-signed-zeros and
-fno-trapping-math are in effect.
The default is -fno-associative-math.
-freciprocal-math
Allow the reciprocal of a value to be used instead of dividing by
the value if this enables optimizations. For example "x / y" can
be replaced with "x * (1/y)", which is useful if "(1/y)" is subject
to common subexpression elimination. Note that this loses
precision and increases the number of flops operating on the value.
The default is -fno-reciprocal-math.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that
arguments and results are not NaNs or +-Infs.
This option is not turned on by any -O option since it can result
in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore the
signedness of zero. IEEE arithmetic specifies the behavior of
distinct +0.0 and -0.0 values, which then prohibits simplification
of expressions such as x+0.0 or 0.0*x (even with
-ffinite-math-only). This option implies that the sign of a zero
result isn't significant.
The default is -fsigned-zeros.
-fno-trapping-math
Compile code assuming that floating-point operations cannot
generate user-visible traps. These traps include division by zero,
overflow, underflow, inexact result and invalid operation. This
option requires that -fno-signaling-nans be in effect. Setting
this option may allow faster code if one relies on "non-stop" IEEE
arithmetic, for example.
This option should never be turned on by any -O option since it can
result in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions.
The default is -ftrapping-math.
-frounding-math
Disable transformations and optimizations that assume default
floating-point rounding behavior. This is round-to-zero for all
floating point to integer conversions, and round-to-nearest for all
other arithmetic truncations. This option should be specified for
programs that change the FP rounding mode dynamically, or that may
be executed with a non-default rounding mode. This option disables
constant folding of floating-point expressions at compile time
(which may be affected by rounding mode) and arithmetic
transformations that are unsafe in the presence of sign-dependent
rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that are affected by rounding mode.
Future versions of GCC may provide finer control of this setting
using C99's "FENV_ACCESS" pragma. This command-line option will be
used to specify the default state for "FENV_ACCESS".
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-
visible traps during floating-point operations. Setting this
option disables optimizations that may change the number of
exceptions visible with signaling NaNs. This option implies
-ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be
defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that affect signaling NaN behavior.
-fsingle-precision-constant
Treat floating-point constants as single precision instead of
implicitly converting them to double-precision constants.
-fcx-limited-range
When enabled, this option states that a range reduction step is not
needed when performing complex division. Also, there is no
checking whether the result of a complex multiplication or division
is "NaN + I*NaN", with an attempt to rescue the situation in that
case. The default is -fno-cx-limited-range, but is enabled by
-ffast-math.
This option controls the default setting of the ISO C99
"CX_LIMITED_RANGE" pragma. Nevertheless, the option applies to all
languages.
-fcx-fortran-rules
Complex multiplication and division follow Fortran rules. Range
reduction is done as part of complex division, but there is no
checking whether the result of a complex multiplication or division
is "NaN + I*NaN", with an attempt to rescue the situation in that
case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve
performance, but are not enabled by any -O options. This section
includes experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can
compile it a second time using -fbranch-probabilities, to improve
optimizations based on the number of times each branch was taken.
When the program compiled with -fprofile-arcs exits it saves arc
execution counts to a file called sourcename.gcda for each source
file. The information in this data file is very dependent on the
structure of the generated code, so you must use the same source
code and the same optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each
JUMP_INSN and CALL_INSN. These can be used to improve
optimization. Currently, they are only used in one place: in
reorg.c, instead of guessing which path a branch is most likely to
take, the REG_BR_PROB values are used to exactly determine which
path is taken more often.
-fprofile-values
If combined with -fprofile-arcs, it adds code so that some data
about values of expressions in the program is gathered.
With -fbranch-probabilities, it reads back the data gathered from
profiling values of expressions for usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
-fvpt
If combined with -fprofile-arcs, it instructs the compiler to add a
code to gather information about values of expressions.
With -fbranch-probabilities, it reads back the data gathered and
actually performs the optimizations based on them. Currently the
optimizations include specialization of division operation using
the knowledge about the value of the denominator.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use
of registers left over after register allocation. This
optimization will most benefit processors with lots of registers.
Depending on the debug information format adopted by the target,
however, it can make debugging impossible, since variables will no
longer stay in a "home register".
Enabled by default with -funroll-loops and -fpeel-loops.
-ftracer
Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function allowing
other optimizations to do better job.
Enabled with -fprofile-use.
-funroll-loops
Unroll loops whose number of iterations can be determined at
compile time or upon entry to the loop. -funroll-loops implies
-frerun-cse-after-loop, -fweb and -frename-registers. It also
turns on complete loop peeling (i.e. complete removal of loops with
small constant number of iterations). This option makes code
larger, and may or may not make it run faster.
Enabled with -fprofile-use.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain
when the loop is entered. This usually makes programs run more
slowly. -funroll-all-loops implies the same options as
-funroll-loops.
-fpeel-loops
Peels loops for which there is enough information that they do not
roll much (from profile feedback). It also turns on complete loop
peeling (i.e. complete removal of loops with small constant number
of iterations).
Enabled with -fprofile-use.
-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop optimizer.
Enabled at level -O1-funswitch-loops
Move branches with loop invariant conditions out of the loop, with
duplicates of the loop on both branches (modified according to
result of the condition).
-ffunction-sections-fdata-sections
Place each function or data item into its own section in the output
file if the target supports arbitrary sections. The name of the
function or the name of the data item determines the section's name
in the output file.
Use these options on systems where the linker can perform
optimizations to improve locality of reference in the instruction
space. Most systems using the ELF object format and SPARC
processors running Solaris 2 have linkers with such optimizations.
AIX may have these optimizations in the future.
Only use these options when there are significant benefits from
doing so. When you specify these options, the assembler and linker
will create larger object and executable files and will also be
slower. You will not be able to use "gprof" on all systems if you
specify this option and you may have problems with debugging if you
specify both this option and -g.
-fbranch-target-load-optimize
Perform branch target register load optimization before prologue /
epilogue threading. The use of target registers can typically be
exposed only during reload, thus hoisting loads out of loops and
doing inter-block scheduling needs a separate optimization pass.
-fbranch-target-load-optimize2
Perform branch target register load optimization after prologue /
epilogue threading.
-fbtr-bb-exclusive
When performing branch target register load optimization, don't
reuse branch target registers in within any basic block.
-fstack-protector
Emit extra code to check for buffer overflows, such as stack
smashing attacks. This is done by adding a guard variable to
functions with vulnerable objects. This includes functions that
call alloca, and functions with buffers larger than 8 bytes. The
guards are initialized when a function is entered and then checked
when the function exits. If a guard check fails, an error message
is printed and the program exits.
-fstack-protector-all
Like -fstack-protector except that all functions are protected.
-fsection-anchors
Try to reduce the number of symbolic address calculations by using
shared "anchor" symbols to address nearby objects. This
transformation can help to reduce the number of GOT entries and GOT
accesses on some targets.
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }
would usually calculate the addresses of all three variables, but
if you compile it with -fsection-anchors, it will access the
variables from a common anchor point instead. The effect is
similar to the following pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
--paramname=value
In some places, GCC uses various constants to control the amount of
optimization that is done. For example, GCC will not inline
functions that contain more than a certain number of instructions.
You can control some of these constants on the command line using
the --param option.
The names of specific parameters, and the meaning of the values,
are tied to the internals of the compiler, and are subject to
change without notice in future releases.
In each case, the value is an integer. The allowable choices for
name are given in the following table:
predictable-branch-outcome
When branch is predicted to be taken with probability lower
than this threshold (in percent), then it is considered well
predictable. The default is 10.
max-crossjump-edges
The maximum number of incoming edges to consider for
crossjumping. The algorithm used by -fcrossjumping is O(N^2)
in the number of edges incoming to each block. Increasing
values mean more aggressive optimization, making the
compilation time increase with probably small improvement in
executable size.
min-crossjump-insns
The minimum number of instructions that must be matched at the
end of two blocks before crossjumping will be performed on
them. This value is ignored in the case where all instructions
in the block being crossjumped from are matched. The default
value is 5.
max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic
blocks instead of jumping. The expansion is relative to a jump
instruction. The default value is 8.
max-goto-duplication-insns
The maximum number of instructions to duplicate to a block that
jumps to a computed goto. To avoid O(N^2) behavior in a number
of passes, GCC factors computed gotos early in the compilation
process, and unfactors them as late as possible. Only computed
jumps at the end of a basic blocks with no more than max-goto-
duplication-insns are unfactored. The default value is 8.
max-delay-slot-insn-search
The maximum number of instructions to consider when looking for
an instruction to fill a delay slot. If more than this
arbitrary number of instructions is searched, the time savings
from filling the delay slot will be minimal so stop searching.
Increasing values mean more aggressive optimization, making the
compilation time increase with probably small improvement in
execution time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of
instructions to consider when searching for a block with valid
live register information. Increasing this arbitrarily chosen
value means more aggressive optimization, increasing the
compilation time. This parameter should be removed when the
delay slot code is rewritten to maintain the control-flow
graph.
max-gcse-memory
The approximate maximum amount of memory that will be allocated
in order to perform the global common subexpression elimination
optimization. If more memory than specified is required, the
optimization will not be done.
max-gcse-insertion-ratio
If the ratio of expression insertions to deletions is larger
than this value for any expression, then RTL PRE will insert or
remove the expression and thus leave partially redundant
computations in the instruction stream. The default value is
20.
max-pending-list-length
The maximum number of pending dependencies scheduling will
allow before flushing the current state and starting over.
Large functions with few branches or calls can create
excessively large lists which needlessly consume memory and
resources.
max-modulo-backtrack-attempts
The maximum number of backtrack attempts the scheduler should
make when modulo scheduling a loop. Larger values can
exponentially increase compilation time.
max-inline-insns-single
Several parameters control the tree inliner used in gcc. This
number sets the maximum number of instructions (counted in
GCC's internal representation) in a single function that the
tree inliner will consider for inlining. This only affects
functions declared inline and methods implemented in a class
declaration (C++). The default value is 400.
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of
functions that would otherwise not be considered for inlining
by the compiler will be investigated. To those functions, a
different (more restrictive) limit compared to functions
declared inline can be applied. The default value is 40.
large-function-insns
The limit specifying really large functions. For functions
larger than this limit after inlining, inlining is constrained
by --paramlarge-function-growth. This parameter is useful
primarily to avoid extreme compilation time caused by non-
linear algorithms used by the back end. The default value is
2700.
large-function-growth
Specifies maximal growth of large function caused by inlining
in percents. The default value is 100 which limits large
function growth to 2.0 times the original size.
large-unit-insns
The limit specifying large translation unit. Growth caused by
inlining of units larger than this limit is limited by --paraminline-unit-growth. For small units this might be too tight
(consider unit consisting of function A that is inline and B
that just calls A three time. If B is small relative to A, the
growth of unit is 300\% and yet such inlining is very sane.
For very large units consisting of small inlineable functions
however the overall unit growth limit is needed to avoid
exponential explosion of code size. Thus for smaller units,
the size is increased to --paramlarge-unit-insns before
applying --paraminline-unit-growth. The default is 10000
inline-unit-growth
Specifies maximal overall growth of the compilation unit caused
by inlining. The default value is 30 which limits unit growth
to 1.3 times the original size.
ipcp-unit-growth
Specifies maximal overall growth of the compilation unit caused
by interprocedural constant propagation. The default value is
10 which limits unit growth to 1.1 times the original size.
large-stack-frame
The limit specifying large stack frames. While inlining the
algorithm is trying to not grow past this limit too much.
Default value is 256 bytes.
large-stack-frame-growth
Specifies maximal growth of large stack frames caused by
inlining in percents. The default value is 1000 which limits
large stack frame growth to 11 times the original size.
max-inline-insns-recursivemax-inline-insns-recursive-auto
Specifies maximum number of instructions out-of-line copy of
self recursive inline function can grow into by performing
recursive inlining.
For functions declared inline --parammax-inline-insns-recursive is taken into account. For function not declared
inline, recursive inlining happens only when -finline-functions
(included in -O3) is enabled and --parammax-inline-insns-recursive-auto is used. The default value is 450.
max-inline-recursive-depthmax-inline-recursive-depth-auto
Specifies maximum recursion depth used by the recursive
inlining.
For functions declared inline --parammax-inline-recursive-depth is taken into account. For function not declared inline,
recursive inlining happens only when -finline-functions
(included in -O3) is enabled and --parammax-inline-recursive-depth-auto is used. The default value is 8.
min-inline-recursive-probability
Recursive inlining is profitable only for function having deep
recursion in average and can hurt for function having little
recursion depth by increasing the prologue size or complexity
of function body to other optimizers.
When profile feedback is available (see -fprofile-generate) the
actual recursion depth can be guessed from probability that
function will recurse via given call expression. This
parameter limits inlining only to call expression whose
probability exceeds given threshold (in percents). The default
value is 10.
early-inlining-insns
Specify growth that early inliner can make. In effect it
increases amount of inlining for code having large abstraction
penalty. The default value is 10.
max-early-inliner-iterationsmax-early-inliner-iterations
Limit of iterations of early inliner. This basically bounds
number of nested indirect calls early inliner can resolve.
Deeper chains are still handled by late inlining.
comdat-sharing-probabilitycomdat-sharing-probability
Probability (in percent) that C++ inline function with comdat
visibility will be shared across multiple compilation units.
The default value is 20.
min-vect-loop-bound
The minimum number of iterations under which a loop will not
get vectorized when -ftree-vectorize is used. The number of
iterations after vectorization needs to be greater than the
value specified by this option to allow vectorization. The
default value is 0.
gcse-cost-distance-ratio
Scaling factor in calculation of maximum distance an expression
can be moved by GCSE optimizations. This is currently
supported only in the code hoisting pass. The bigger the
ratio, the more aggressive code hoisting will be with simple
expressions, i.e., the expressions that have cost less than
gcse-unrestricted-cost. Specifying 0 will disable hoisting of
simple expressions. The default value is 10.
gcse-unrestricted-cost
Cost, roughly measured as the cost of a single typical machine
instruction, at which GCSE optimizations will not constrain the
distance an expression can travel. This is currently supported
only in the code hoisting pass. The lesser the cost, the more
aggressive code hoisting will be. Specifying 0 will allow all
expressions to travel unrestricted distances. The default
value is 3.
max-hoist-depth
The depth of search in the dominator tree for expressions to
hoist. This is used to avoid quadratic behavior in hoisting
algorithm. The value of 0 will avoid limiting the search, but
may slow down compilation of huge functions. The default value
is 30.
max-tail-merge-comparisons
The maximum amount of similar bbs to compare a bb with. This
is used to avoid quadratic behavior in tree tail merging. The
default value is 10.
max-tail-merge-iterations
The maximum amount of iterations of the pass over the function.
This is used to limit compilation time in tree tail merging.
The default value is 2.
max-unrolled-insns
The maximum number of instructions that a loop should have if
that loop is unrolled, and if the loop is unrolled, it
determines how many times the loop code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by probabilities of
their execution that a loop should have if that loop is
unrolled, and if the loop is unrolled, it determines how many
times the loop code is unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop should have if
that loop is peeled, and if the loop is peeled, it determines
how many times the loop code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.
max-completely-peel-times
The maximum number of iterations of a loop to be suitable for
complete peeling.
max-completely-peel-loop-nest-depth
The maximum depth of a loop nest suitable for complete peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a single loop.
lim-expensive
The minimum cost of an expensive expression in the loop
invariant motion.
iv-consider-all-candidates-bound
Bound on number of candidates for induction variables below
that all candidates are considered for each use in induction
variable optimizations. Only the most relevant candidates are
considered if there are more candidates, to avoid quadratic
time complexity.
iv-max-considered-uses
The induction variable optimizations give up on loops that
contain more induction variable uses.
iv-always-prune-cand-set-bound
If number of candidates in the set is smaller than this value,
we always try to remove unnecessary ivs from the set during its
optimization when a new iv is added to the set.
scev-max-expr-size
Bound on size of expressions used in the scalar evolutions
analyzer. Large expressions slow the analyzer.
scev-max-expr-complexity
Bound on the complexity of the expressions in the scalar
evolutions analyzer. Complex expressions slow the analyzer.
omega-max-vars
The maximum number of variables in an Omega constraint system.
The default value is 128.
omega-max-geqs
The maximum number of inequalities in an Omega constraint
system. The default value is 256.
omega-max-eqs
The maximum number of equalities in an Omega constraint system.
The default value is 128.
omega-max-wild-cards
The maximum number of wildcard variables that the Omega solver
will be able to insert. The default value is 18.
omega-hash-table-size
The size of the hash table in the Omega solver. The default
value is 550.
omega-max-keys
The maximal number of keys used by the Omega solver. The
default value is 500.
omega-eliminate-redundant-constraints
When set to 1, use expensive methods to eliminate all redundant
constraints. The default value is 0.
vect-max-version-for-alignment-checks
The maximum number of run-time checks that can be performed
when doing loop versioning for alignment in the vectorizer.
See option ftree-vect-loop-version for more information.
vect-max-version-for-alias-checks
The maximum number of run-time checks that can be performed
when doing loop versioning for alias in the vectorizer. See
option ftree-vect-loop-version for more information.
max-iterations-to-track
The maximum number of iterations of a loop the brute force
algorithm for analysis of # of iterations of the loop tries to
evaluate.
hot-bb-count-fraction
Select fraction of the maximal count of repetitions of basic
block in program given basic block needs to have to be
considered hot.
hot-bb-frequency-fraction
Select fraction of the entry block frequency of executions of
basic block in function given basic block needs to have to be
considered hot.
max-predicted-iterations
The maximum number of loop iterations we predict statically.
This is useful in cases where function contain single loop with
known bound and other loop with unknown. We predict the known
number of iterations correctly, while the unknown number of
iterations average to roughly 10. This means that the loop
without bounds would appear artificially cold relative to the
other one.
align-threshold
Select fraction of the maximal frequency of executions of basic
block in function given basic block will get aligned.
align-loop-iterations
A loop expected to iterate at lest the selected number of
iterations will get aligned.
tracer-dynamic-coveragetracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given
percentage of executed instructions is covered. This limits
unnecessary code size expansion.
The tracer-dynamic-coverage-feedback is used only when profile
feedback is available. The real profiles (as opposed to
statically estimated ones) are much less balanced allowing the
threshold to be larger value.
tracer-max-code-growth
Stop tail duplication once code growth has reached given
percentage. This is rather hokey argument, as most of the
duplicates will be eliminated later in cross jumping, so it may
be set to much higher values than is the desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge
is less than this threshold (in percent).
tracer-min-branch-ratiotracer-min-branch-ratio-feedback
Stop forward growth if the best edge do have probability lower
than this threshold.
Similarly to tracer-dynamic-coverage two values are present,
one for compilation for profile feedback and one for
compilation without. The value for compilation with profile
feedback needs to be more conservative (higher) in order to
make tracer effective.
max-cse-path-length
Maximum number of basic blocks on path that cse considers. The
default is 10.
max-cse-insns
The maximum instructions CSE process before flushing. The
default is 1000.
ggc-min-expand
GCC uses a garbage collector to manage its own memory
allocation. This parameter specifies the minimum percentage by
which the garbage collector's heap should be allowed to expand
between collections. Tuning this may improve compilation
speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of
100% when RAM >= 1GB. If "getrlimit" is available, the notion
of "RAM" is the smallest of actual RAM and "RLIMIT_DATA" or
"RLIMIT_AS". If GCC is not able to calculate RAM on a
particular platform, the lower bound of 30% is used. Setting
this parameter and ggc-min-heapsize to zero causes a full
collection to occur at every opportunity. This is extremely
slow, but can be useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins
bothering to collect garbage. The first collection occurs
after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation speed,
and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit
that tries to ensure that RLIMIT_DATA or RLIMIT_AS are not
exceeded, but with a lower bound of 4096 (four megabytes) and
an upper bound of 131072 (128 megabytes). If GCC is not able
to calculate RAM on a particular platform, the lower bound is
used. Setting this parameter very large effectively disables
garbage collection. Setting this parameter and ggc-min-expand
to zero causes a full collection to occur at every opportunity.
max-reload-search-insns
The maximum number of instruction reload should look backward
for equivalent register. Increasing values mean more
aggressive optimization, making the compilation time increase
with probably slightly better performance. The default value
is 100.
max-cselib-memory-locations
The maximum number of memory locations cselib should take into
account. Increasing values mean more aggressive optimization,
making the compilation time increase with probably slightly
better performance. The default value is 500.
reorder-blocks-duplicatereorder-blocks-duplicate-feedback
Used by basic block reordering pass to decide whether to use
unconditional branch or duplicate the code on its destination.
Code is duplicated when its estimated size is smaller than this
value multiplied by the estimated size of unconditional jump in
the hot spots of the program.
The reorder-block-duplicate-feedback is used only when profile
feedback is available and may be set to higher values than
reorder-block-duplicate since information about the hot spots
is more accurate.
max-sched-ready-insns
The maximum number of instructions ready to be issued the
scheduler should consider at any given time during the first
scheduling pass. Increasing values mean more thorough
searches, making the compilation time increase with probably
little benefit. The default value is 100.
max-sched-region-blocks
The maximum number of blocks in a region to be considered for
interblock scheduling. The default value is 10.
max-pipeline-region-blocks
The maximum number of blocks in a region to be considered for
pipelining in the selective scheduler. The default value is
15.
max-sched-region-insns
The maximum number of insns in a region to be considered for
interblock scheduling. The default value is 100.
max-pipeline-region-insns
The maximum number of insns in a region to be considered for
pipelining in the selective scheduler. The default value is
200.
min-spec-prob
The minimum probability (in percents) of reaching a source
block for interblock speculative scheduling. The default value
is 40.
max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend regions.
0 - disable region extension, N - do at most N iterations. The
default value is 0.
max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered for
speculative motion. The default value is 3.
sched-spec-prob-cutoff
The minimal probability of speculation success (in percents),
so that speculative insn will be scheduled. The default value
is 40.
sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and load
targeting same memory locations. The default value is 1.
selsched-max-lookahead
The maximum size of the lookahead window of selective
scheduling. It is a depth of search for available
instructions. The default value is 50.
selsched-max-sched-times
The maximum number of times that an instruction will be
scheduled during selective scheduling. This is the limit on
the number of iterations through which the instruction may be
pipelined. The default value is 2.
selsched-max-insns-to-rename
The maximum number of best instructions in the ready list that
are considered for renaming in the selective scheduler. The
default value is 2.
sms-min-sc
The minimum value of stage count that swing modulo scheduler
will generate. The default value is 2.
max-last-value-rtl
The maximum size measured as number of RTLs that can be
recorded in an expression in combiner for a pseudo register as
last known value of that register. The default is 10000.
integer-share-limit
Small integer constants can use a shared data structure,
reducing the compiler's memory usage and increasing its speed.
This sets the maximum value of a shared integer constant. The
default value is 256.
min-virtual-mappings
Specifies the minimum number of virtual mappings in the
incremental SSA updater that should be registered to trigger
the virtual mappings heuristic defined by virtual-mappings-
ratio. The default value is 100.
virtual-mappings-ratio
If the number of virtual mappings is virtual-mappings-ratio
bigger than the number of virtual symbols to be updated, then
the incremental SSA updater switches to a full update for those
symbols. The default ratio is 3.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that will receive
stack smashing protection when -fstack-protection is used.
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to
be duplicated when threading jumps.
max-fields-for-field-sensitive
Maximum number of fields in a structure we will treat in a
field sensitive manner during pointer analysis. The default is
zero for -O0, and -O1 and 100 for -Os, -O2, and -O3.
prefetch-latency
Estimate on average number of instructions that are executed
before prefetch finishes. The distance we prefetch ahead is
proportional to this constant. Increasing this number may also
lead to less streams being prefetched (see simultaneous-prefetches).
simultaneous-prefetches
Maximum number of prefetches that can run at the same time.
l1-cache-line-size
The size of cache line in L1 cache, in bytes.
l1-cache-size
The size of L1 cache, in kilobytes.
l2-cache-size
The size of L2 cache, in kilobytes.
min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions and the
number of prefetches to enable prefetching in a loop.
prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions and the
number of memory references to enable prefetching in a loop.
use-canonical-types
Whether the compiler should use the "canonical" type system.
By default, this should always be 1, which uses a more
efficient internal mechanism for comparing types in C++ and
Objective-C++. However, if bugs in the canonical type system
are causing compilation failures, set this value to 0 to
disable canonical types.
switch-conversion-max-branch-ratio
Switch initialization conversion will refuse to create arrays
that are bigger than switch-conversion-max-branch-ratio times
the number of branches in the switch.
max-partial-antic-length
Maximum length of the partial antic set computed during the
tree partial redundancy elimination optimization (-ftree-pre)
when optimizing at -O3 and above. For some sorts of source
code the enhanced partial redundancy elimination optimization
can run away, consuming all of the memory available on the host
machine. This parameter sets a limit on the length of the sets
that are computed, which prevents the runaway behavior.
Setting a value of 0 for this parameter will allow an unlimited
set length.
sccvn-max-scc-size
Maximum size of a strongly connected component (SCC) during
SCCVN processing. If this limit is hit, SCCVN processing for
the whole function will not be done and optimizations depending
on it will be disabled. The default maximum SCC size is 10000.
ira-max-loops-num
IRA uses regional register allocation by default. If a
function contains more loops than the number given by this
parameter, only at most the given number of the most
frequently-executed loops form regions for regional register
allocation. The default value of the parameter is 100.
ira-max-conflict-table-size
Although IRA uses a sophisticated algorithm to compress the
conflict table, the table can still require excessive amounts
of memory for huge functions. If the conflict table for a
function could be more than the size in MB given by this
parameter, the register allocator instead uses a faster,
simpler, and lower-quality algorithm that does not require
building a pseudo-register conflict table. The default value
of the parameter is 2000.
ira-loop-reserved-regs
IRA can be used to evaluate more accurate register pressure in
loops for decisions to move loop invariants (see -O3). The
number of available registers reserved for some other purposes
is given by this parameter. The default value of the parameter
is 2, which is the minimal number of registers needed by
typical instructions. This value is the best found from
numerous experiments.
loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive, both in
compilation time and in amount of needed compile-time memory,
with very large loops. Loops with more basic blocks than this
parameter won't have loop invariant motion optimization
performed on them. The default value of the parameter is 1000
for -O1 and 10000 for -O2 and above.
loop-max-datarefs-for-datadeps
Building data dapendencies is expensive for very large loops.
This parameter limits the number of data references in loops
that are considered for data dependence analysis. These large
loops will not be handled then by the optimizations using loop
data dependencies. The default value is 1000.
max-vartrack-size
Sets a maximum number of hash table slots to use during
variable tracking dataflow analysis of any function. If this
limit is exceeded with variable tracking at assignments
enabled, analysis for that function is retried without it,
after removing all debug insns from the function. If the limit
is exceeded even without debug insns, var tracking analysis is
completely disabled for the function. Setting the parameter to
zero makes it unlimited.
max-vartrack-expr-depth
Sets a maximum number of recursion levels when attempting to
map variable names or debug temporaries to value expressions.
This trades compilation time for more complete debug
information. If this is set too low, value expressions that
are available and could be represented in debug information may
end up not being used; setting this higher may enable the
compiler to find more complex debug expressions, but compile
time and memory use may grow. The default is 12.
min-nondebug-insn-uid
Use uids starting at this parameter for nondebug insns. The
range below the parameter is reserved exclusively for debug
insns created by -fvar-tracking-assignments, but debug insns
may get (non-overlapping) uids above it if the reserved range
is exhausted.
ipa-sra-ptr-growth-factor
IPA-SRA will replace a pointer to an aggregate with one or more
new parameters only when their cumulative size is less or equal
to ipa-sra-ptr-growth-factor times the size of the original
pointer parameter.
tm-max-aggregate-size
When making copies of thread-local variables in a transaction,
this parameter specifies the size in bytes after which
variables will be saved with the logging functions as opposed
to save/restore code sequence pairs. This option only applies
when using -fgnu-tm.
graphite-max-nb-scop-params
To avoid exponential effects in the Graphite loop transforms,
the number of parameters in a Static Control Part (SCoP) is
bounded. The default value is 10 parameters. A variable whose
value is unknown at compilation time and defined outside a SCoP
is a parameter of the SCoP.
graphite-max-bbs-per-function
To avoid exponential effects in the detection of SCoPs, the
size of the functions analyzed by Graphite is bounded. The
default value is 100 basic blocks.
loop-block-tile-size
Loop blocking or strip mining transforms, enabled with
-floop-block or -floop-strip-mine, strip mine each loop in the
loop nest by a given number of iterations. The strip length
can be changed using the loop-block-tile-size parameter. The
default value is 51 iterations.
ipa-cp-value-list-size
IPA-CP attempts to track all possible values and types passed
to a function's parameter in order to propagate them and
perform devirtualization. ipa-cp-value-list-size is the
maximum number of values and types it stores per one formal
parameter of a function.
lto-partitions
Specify desired number of partitions produced during WHOPR
compilation. The number of partitions should exceed the number
of CPUs used for compilation. The default value is 32.
lto-minpartition
Size of minimal partition for WHOPR (in estimated
instructions). This prevents expenses of splitting very small
programs into too many partitions.
cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for suggestions
when C++ name lookup fails for an identifier. The default is
1000.
sink-frequency-threshold
The maximum relative execution frequency (in percents) of the
target block relative to a statement's original block to allow
statement sinking of a statement. Larger numbers result in
more aggressive statement sinking. The default value is 75. A
small positive adjustment is applied for statements with memory
operands as those are even more profitable so sink.
max-stores-to-sink
The maximum number of conditional stores paires that can be
sunk. Set to 0 if either vectorization (-ftree-vectorize) or
if-conversion (-ftree-loop-if-convert) is disabled. The
default is 2.
allow-load-data-races
Allow optimizers to introduce new data races on loads. Set to
1 to allow, otherwise to 0. This option is enabled by default
unless implicitly set by the -fmemory-model= option.
allow-store-data-races
Allow optimizers to introduce new data races on stores. Set to
1 to allow, otherwise to 0. This option is enabled by default
unless implicitly set by the -fmemory-model= option.
allow-packed-load-data-races
Allow optimizers to introduce new data races on packed data
loads. Set to 1 to allow, otherwise to 0. This option is
enabled by default unless implicitly set by the -fmemory-model=
option.
allow-packed-store-data-races
Allow optimizers to introduce new data races on packed data
stores. Set to 1 to allow, otherwise to 0. This option is
enabled by default unless implicitly set by the -fmemory-model=
option.
case-values-threshold
The smallest number of different values for which it is best to
use a jump-table instead of a tree of conditional branches. If
the value is 0, use the default for the machine. The default
is 0.
tree-reassoc-width
Set the maximum number of instructions executed in parallel in
reassociated tree. This parameter overrides target dependent
heuristics used by default if has non zero value.
OptionsControllingthePreprocessor
These options control the C preprocessor, which is run on each C source
file before actual compilation.
If you use the -E option, nothing is done except preprocessing. Some
of these options make sense only together with -E because they cause
the preprocessor output to be unsuitable for actual compilation.
-Wp,option
You can use -Wp,option to bypass the compiler driver and pass
option directly through to the preprocessor. If option contains
commas, it is split into multiple options at the commas. However,
many options are modified, translated or interpreted by the
compiler driver before being passed to the preprocessor, and -Wp
forcibly bypasses this phase. The preprocessor's direct interface
is undocumented and subject to change, so whenever possible you
should avoid using -Wp and let the driver handle the options
instead.
-Xpreprocessoroption
Pass option as an option to the preprocessor. You can use this to
supply system-specific preprocessor options that GCC does not know
how to recognize.
If you want to pass an option that takes an argument, you must use
-Xpreprocessor twice, once for the option and once for the
argument.
-Dname
Predefine name as a macro, with definition 1.
-Dname=definition
The contents of definition are tokenized and processed as if they
appeared during translation phase three in a #define directive. In
particular, the definition will be truncated by embedded newline
characters.
If you are invoking the preprocessor from a shell or shell-like
program you may need to use the shell's quoting syntax to protect
characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line,
write its argument list with surrounding parentheses before the
equals sign (if any). Parentheses are meaningful to most shells,
so you will need to quote the option. With sh and csh,
-D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on the
command line. All -imacrosfile and -includefile options are
processed after all -D and -U options.
-Uname
Cancel any previous definition of name, either built in or provided
with a -D option.
-undef
Do not predefine any system-specific or GCC-specific macros. The
standard predefined macros remain defined.
-Idir
Add the directory dir to the list of directories to be searched for
header files. Directories named by -I are searched before the
standard system include directories. If the directory dir is a
standard system include directory, the option is ignored to ensure
that the default search order for system directories and the
special treatment of system headers are not defeated . If dir
begins with "=", then the "=" will be replaced by the sysroot
prefix; see --sysroot and -isysroot.
-ofile
Write output to file. This is the same as specifying file as the
second non-option argument to cpp. gcc has a different
interpretation of a second non-option argument, so you must use -o
to specify the output file.
-Wall
Turns on all optional warnings which are desirable for normal code.
At present this is -Wcomment, -Wtrigraphs, -Wmultichar and a
warning about integer promotion causing a change of sign in "#if"
expressions. Note that many of the preprocessor's warnings are on
by default and have no options to control them.
-Wcomment-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment,
or whenever a backslash-newline appears in a // comment. (Both
forms have the same effect.)
-Wtrigraphs
Most trigraphs in comments cannot affect the meaning of the
program. However, a trigraph that would form an escaped newline
(??/ at the end of a line) can, by changing where the comment
begins or ends. Therefore, only trigraphs that would form escaped
newlines produce warnings inside a comment.
This option is implied by -Wall. If -Wall is not given, this
option is still enabled unless trigraphs are enabled. To get
trigraph conversion without warnings, but get the other -Wall
warnings, use -trigraphs-Wall-Wno-trigraphs.
-Wtraditional
Warn about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO C constructs that have
no traditional C equivalent, and problematic constructs which
should be avoided.
-Wundef
Warn whenever an identifier which is not a macro is encountered in
an #if directive, outside of defined. Such identifiers are
replaced with zero.
-Wunused-macros
Warn about macros defined in the main file that are unused. A
macro is used if it is expanded or tested for existence at least
once. The preprocessor will also warn if the macro has not been
used at the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros
defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped
conditional blocks, then CPP will report it as unused. To avoid
the warning in such a case, you might improve the scope of the
macro's definition by, for example, moving it into the first
skipped block. Alternatively, you could provide a dummy use with
something like:
#if defined the_macro_causing_the_warning
#endif
-Wendif-labels
Warn whenever an #else or an #endif are followed by text. This
usually happens in code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments, but often are not
in older programs. This warning is on by default.
-Werror
Make all warnings into hard errors. Source code which triggers
warnings will be rejected.
-Wsystem-headers
Issue warnings for code in system headers. These are normally
unhelpful in finding bugs in your own code, therefore suppressed.
If you are responsible for the system library, you may want to see
them.
-w Suppress all warnings, including those which GNU CPP issues by
default.
-pedantic
Issue all the mandatory diagnostics listed in the C standard. Some
of them are left out by default, since they trigger frequently on
harmless code.
-pedantic-errors
Issue all the mandatory diagnostics, and make all mandatory
diagnostics into errors. This includes mandatory diagnostics that
GCC issues without -pedantic but treats as warnings.
-M Instead of outputting the result of preprocessing, output a rule
suitable for make describing the dependencies of the main source
file. The preprocessor outputs one make rule containing the object
file name for that source file, a colon, and the names of all the
included files, including those coming from -include or -imacros
command line options.
Unless specified explicitly (with -MT or -MQ), the object file name
consists of the name of the source file with any suffix replaced
with object file suffix and with any leading directory parts
removed. If there are many included files then the rule is split
into several lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such
as -dM. To avoid mixing such debug output with the dependency
rules you should explicitly specify the dependency output file with
-MF, or use an environment variable like DEPENDENCIES_OUTPUT.
Debug output will still be sent to the regular output stream as
normal.
Passing -M to the driver implies -E, and suppresses warnings with
an implicit -w.
-MM Like -M but do not mention header files that are found in system
header directories, nor header files that are included, directly or
indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in
an #include directive does not in itself determine whether that
header will appear in -MM dependency output. This is a slight
change in semantics from GCC versions 3.0 and earlier.
-MFfile
When used with -M or -MM, specifies a file to write the
dependencies to. If no -MF switch is given the preprocessor sends
the rules to the same place it would have sent preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the
default dependency output file.
-MG In conjunction with an option such as -M requesting dependency
generation, -MG assumes missing header files are generated files
and adds them to the dependency list without raising an error. The
dependency filename is taken directly from the "#include" directive
without prepending any path. -MG also suppresses preprocessed
output, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each dependency
other than the main file, causing each to depend on nothing. These
dummy rules work around errors make gives if you remove header
files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MTtarget
Change the target of the rule emitted by dependency generation. By
default CPP takes the name of the main input file, deletes any
directory components and any file suffix such as .c, and appends
the platform's usual object suffix. The result is the target.
An -MT option will set the target to be exactly the string you
specify. If you want multiple targets, you can specify them as a
single argument to -MT, or use multiple -MT options.
For example, -MT'$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQtarget
Same as -MT, but it quotes any characters which are special to
Make. -MQ'$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given
with -MQ.
-MD-MD is equivalent to -M-MFfile, except that -E is not implied.
The driver determines file based on whether an -o option is given.
If it is, the driver uses its argument but with a suffix of .d,
otherwise it takes the name of the input file, removes any
directory components and suffix, and applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is understood
to specify the dependency output file, but if used without -E, each
-o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency
output file as a side-effect of the compilation process.
-MMD
Like -MD except mention only user header files, not system header
files.
-fpch-deps
When using precompiled headers, this flag will cause the
dependency-output flags to also list the files from the precompiled
header's dependencies. If not specified only the precompiled
header would be listed and not the files that were used to create
it because those files are not consulted when a precompiled header
is used.
-fpch-preprocess
This option allows use of a precompiled header together with -E.
It inserts a special "#pragma", "#pragma GCC pch_preprocess
"filename"" in the output to mark the place where the precompiled
header was found, and its filename. When -fpreprocessed is in use,
GCC recognizes this "#pragma" and loads the PCH.
This option is off by default, because the resulting preprocessed
output is only really suitable as input to GCC. It is switched on
by -save-temps.
You should not write this "#pragma" in your own code, but it is
safe to edit the filename if the PCH file is available in a
different location. The filename may be absolute or it may be
relative to GCC's current directory.
-xc-xc++-xobjective-c-xassembler-with-cpp
Specify the source language: C, C++, Objective-C, or assembly.
This has nothing to do with standards conformance or extensions; it
merely selects which base syntax to expect. If you give none of
these options, cpp will deduce the language from the extension of
the source file: .c, .cc, .m, or .S. Some other common extensions
for C++ and assembly are also recognized. If cpp does not
recognize the extension, it will treat the file as C; this is the
most generic mode.
Note: Previous versions of cpp accepted a -lang option which
selected both the language and the standards conformance level.
This option has been removed, because it conflicts with the -l
option.
-std=standard-ansi
Specify the standard to which the code should conform. Currently
CPP knows about C and C++ standards; others may be added in the
future.
standard may be one of:
"c90"
"c89"
"iso9899:1990"
The ISO C standard from 1990. c90 is the customary shorthand
for this version of the standard.
The -ansi option is equivalent to -std=c90.
"iso9899:199409"
The 1990 C standard, as amended in 1994.
"iso9899:1999"
"c99"
"iso9899:199x"
"c9x"
The revised ISO C standard, published in December 1999. Before
publication, this was known as C9X.
"iso9899:2011"
"c11"
"c1x"
The revised ISO C standard, published in December 2011. Before
publication, this was known as C1X.
"gnu90"
"gnu89"
The 1990 C standard plus GNU extensions. This is the default.
"gnu99"
"gnu9x"
The 1999 C standard plus GNU extensions.
"gnu11"
"gnu1x"
The 2011 C standard plus GNU extensions.
"c++98"
The 1998 ISO C++ standard plus amendments.
"gnu++98"
The same as -std=c++98 plus GNU extensions. This is the
default for C++ code.
-I- Split the include path. Any directories specified with -I options
before -I- are searched only for headers requested with
"#include "file""; they are not searched for "#include <file>". If
additional directories are specified with -I options after the -I-,
those directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current
file directory as the first search directory for "#include "file"".
This option has been deprecated.
-nostdinc
Do not search the standard system directories for header files.
Only the directories you have specified with -I options (and the
directory of the current file, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C++-specific standard
directories, but do still search the other standard directories.
(This option is used when building the C++ library.)
-includefile
Process file as if "#include "file"" appeared as the first line of
the primary source file. However, the first directory searched for
file is the preprocessor's working directory insteadof the
directory containing the main source file. If not found there, it
is searched for in the remainder of the "#include "..."" search
chain as normal.
If multiple -include options are given, the files are included in
the order they appear on the command line.
-imacrosfile
Exactly like -include, except that any output produced by scanning
file is thrown away. Macros it defines remain defined. This
allows you to acquire all the macros from a header without also
processing its declarations.
All files specified by -imacros are processed before all files
specified by -include.
-idirafterdir
Search dir for header files, but do it after all directories
specified with -I and the standard system directories have been
exhausted. dir is treated as a system include directory. If dir
begins with "=", then the "=" will be replaced by the sysroot
prefix; see --sysroot and -isysroot.
-iprefixprefix
Specify prefix as the prefix for subsequent -iwithprefix options.
If the prefix represents a directory, you should include the final
/.
-iwithprefixdir-iwithprefixbeforedir
Append dir to the prefix specified previously with -iprefix, and
add the resulting directory to the include search path.
-iwithprefixbefore puts it in the same place -I would; -iwithprefix
puts it where -idirafter would.
-isysrootdir
This option is like the --sysroot option, but applies only to
header files (except for Darwin targets, where it applies to both
header files and libraries). See the --sysroot option for more
information.
-imultilibdir
Use dir as a subdirectory of the directory containing target-
specific C++ headers.
-isystemdir
Search dir for header files, after all directories specified by -I
but before the standard system directories. Mark it as a system
directory, so that it gets the same special treatment as is applied
to the standard system directories. If dir begins with "=", then
the "=" will be replaced by the sysroot prefix; see --sysroot and
-isysroot.
-iquotedir
Search dir only for header files requested with "#include "file"";
they are not searched for "#include <file>", before all directories
specified by -I and before the standard system directories. If dir
begins with "=", then the "=" will be replaced by the sysroot
prefix; see --sysroot and -isysroot.
-fdirectives-only
When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed options.
With -E, preprocessing is limited to the handling of directives
such as "#define", "#ifdef", and "#error". Other preprocessor
operations, such as macro expansion and trigraph conversion are not
performed. In addition, the -dD option is implicitly enabled.
With -fpreprocessed, predefinition of command line and most builtin
macros is disabled. Macros such as "__LINE__", which are
contextually dependent, are handled normally. This enables
compilation of files previously preprocessed with "-E
-fdirectives-only".
With both -E and -fpreprocessed, the rules for -fpreprocessed take
precedence. This enables full preprocessing of files previously
preprocessed with "-E -fdirectives-only".
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names in identifiers. This option is
experimental; in a future version of GCC, it will be enabled by
default for C99 and C++.
-fpreprocessed
Indicate to the preprocessor that the input file has already been
preprocessed. This suppresses things like macro expansion,
trigraph conversion, escaped newline splicing, and processing of
most directives. The preprocessor still recognizes and removes
comments, so that you can pass a file preprocessed with -C to the
compiler without problems. In this mode the integrated
preprocessor is little more than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the
extensions .i, .ii or .mi. These are the extensions that GCC uses
for preprocessed files created by -save-temps.
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor
report correct column numbers in warnings or errors, even if tabs
appear on the line. If the value is less than 1 or greater than
100, the option is ignored. The default is 8.
-fdebug-cpp
This option is only useful for debugging GCC. When used with -E,
dumps debugging information about location maps. Every token in
the output is preceded by the dump of the map its location belongs
to. The dump of the map holding the location of a token would be:
{"P":F</file/path>;"F":F</includer/path>;"L":<line_num>;"C":<col_num>;"S":<system_header_p>;"M":<map_address>;"E":<macro_expansion_p>,"loc":<location>}
When used without -E, this option has no effect.
-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This allows the
compiler to emit diagnostic about the current macro expansion stack
when a compilation error occurs in a macro expansion. Using this
option makes the preprocessor and the compiler consume more memory.
The level parameter can be used to choose the level of precision of
token location tracking thus decreasing the memory consumption if
necessary. Value 0 of level de-activates this option just as if no
-ftrack-macro-expansion was present on the command line. Value 1
tracks tokens locations in a degraded mode for the sake of minimal
memory overhead. In this mode all tokens resulting from the
expansion of an argument of a function-like macro have the same
location. Value 2 tracks tokens locations completely. This value is
the most memory hungry. When this option is given no argument, the
default parameter value is 2.
-fexec-charset=charset
Set the execution character set, used for string and character
constants. The default is UTF-8. charset can be any encoding
supported by the system's "iconv" library routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide string and
character constants. The default is UTF-32 or UTF-16, whichever
corresponds to the width of "wchar_t". As with -fexec-charset,
charset can be any encoding supported by the system's "iconv"
library routine; however, you will have problems with encodings
that do not fit exactly in "wchar_t".
-finput-charset=charset
Set the input character set, used for translation from the
character set of the input file to the source character set used by
GCC. If the locale does not specify, or GCC cannot get this
information from the locale, the default is UTF-8. This can be
overridden by either the locale or this command line option.
Currently the command line option takes precedence if there's a
conflict. charset can be any encoding supported by the system's
"iconv" library routine.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that
will let the compiler know the current working directory at the
time of preprocessing. When this option is enabled, the
preprocessor will emit, after the initial linemarker, a second
linemarker with the current working directory followed by two
slashes. GCC will use this directory, when it's present in the
preprocessed input, as the directory emitted as the current working
directory in some debugging information formats. This option is
implicitly enabled if debugging information is enabled, but this
can be inhibited with the negated form -fno-working-directory. If
the -P flag is present in the command line, this option has no
effect, since no "#line" directives are emitted whatsoever.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary
if diagnostics are being scanned by a program that does not
understand the column numbers, such as dejagnu.
-Apredicate=answer
Make an assertion with the predicate predicate and answer answer.
This form is preferred to the older form -Apredicate(answer),
which is still supported, because it does not use shell special
characters.
-A-predicate=answer
Cancel an assertion with the predicate predicate and answer answer.
-dCHARSCHARS is a sequence of one or more of the following characters, and
must not be preceded by a space. Other characters are interpreted
by the compiler proper, or reserved for future versions of GCC, and
so are silently ignored. If you specify characters whose behavior
conflicts, the result is undefined.
M Instead of the normal output, generate a list of #define
directives for all the macros defined during the execution of
the preprocessor, including predefined macros. This gives you
a way of finding out what is predefined in your version of the
preprocessor. Assuming you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
will show all the predefined macros.
If you use -dM without the -E option, -dM is interpreted as a
synonym for -fdump-rtl-mach.
D Like M except in two respects: it does not include the
predefined macros, and it outputs both the #define directives
and the result of preprocessing. Both kinds of output go to
the standard output file.
N Like D, but emit only the macro names, not their expansions.
I Output #include directives in addition to the result of
preprocessing.
U Like D except that only macros that are expanded, or whose
definedness is tested in preprocessor directives, are output;
the output is delayed until the use or test of the macro; and
#undef directives are also output for macros tested but
undefined at the time.
-P Inhibit generation of linemarkers in the output from the
preprocessor. This might be useful when running the preprocessor
on something that is not C code, and will be sent to a program
which might be confused by the linemarkers.
-C Do not discard comments. All comments are passed through to the
output file, except for comments in processed directives, which are
deleted along with the directive.
You should be prepared for side effects when using -C; it causes
the preprocessor to treat comments as tokens in their own right.
For example, comments appearing at the start of what would be a
directive line have the effect of turning that line into an
ordinary source line, since the first token on the line is no
longer a #.
-CC Do not discard comments, including during macro expansion. This is
like -C, except that comments contained within macros are also
passed through to the output file where the macro is expanded.
In addition to the side-effects of the -C option, the -CC option
causes all C++-style comments inside a macro to be converted to
C-style comments. This is to prevent later use of that macro from
inadvertently commenting out the remainder of the source line.
The -CC option is generally used to support lint comments.
-traditional-cpp
Try to imitate the behavior of old-fashioned C preprocessors, as
opposed to ISO C preprocessors.
-trigraphs
Process trigraph sequences. These are three-character sequences,
all starting with ??, that are defined by ISO C to stand for single
characters. For example, ??/ stands for \, so '??/n' is a
character constant for a newline. By default, GCC ignores
trigraphs, but in standard-conforming modes it converts them. See
the -std and -ansi options.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
-remap
Enable special code to work around file systems which only permit
very short file names, such as MS-DOS.
--help--target-help
Print text describing all the command line options instead of
preprocessing anything.
-v Verbose mode. Print out GNU CPP's version number at the beginning
of execution, and report the final form of the include path.
-H Print the name of each header file used, in addition to other
normal activities. Each name is indented to show how deep in the
#include stack it is. Precompiled header files are also printed,
even if they are found to be invalid; an invalid precompiled header
file is printed with ...x and a valid one with ...! .
-version--version
Print out GNU CPP's version number. With one dash, proceed to
preprocess as normal. With two dashes, exit immediately.
PassingOptionstotheAssembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains
commas, it is split into multiple options at the commas.
-Xassembleroption
Pass option as an option to the assembler. You can use this to
supply system-specific assembler options that GCC does not know how
to recognize.
If you want to pass an option that takes an argument, you must use
-Xassembler twice, once for the option and once for the argument.
OptionsforLinking
These options come into play when the compiler links object files into
an executable output file. They are meaningless if the compiler is not
doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is
considered to name an object file or library. (Object files are
distinguished from libraries by the linker according to the file
contents.) If linking is done, these object files are used as
input to the linker.
-c-S-E If any of these options is used, then the linker is not run, and
object file names should not be used as arguments.
-llibrary-llibrary
Search the library named library when linking. (The second
alternative with the library as a separate argument is only for
POSIX compliance and is not recommended.)
It makes a difference where in the command you write this option;
the linker searches and processes libraries and object files in the
order they are specified. Thus, foo.o-lzbar.o searches library z
after file foo.o but before bar.o. If bar.o refers to functions in
z, those functions may not be loaded.
The linker searches a standard list of directories for the library,
which is actually a file named liblibrary.a. The linker then uses
this file as if it had been specified precisely by name.
The directories searched include several standard system
directories plus any that you specify with -L.
Normally the files found this way are library files---archive files
whose members are object files. The linker handles an archive file
by scanning through it for members which define symbols that have
so far been referenced but not defined. But if the file that is
found is an ordinary object file, it is linked in the usual
fashion. The only difference between using an -l option and
specifying a file name is that -l surrounds library with lib and .a
and searches several directories.
-lobjc
You need this special case of the -l option in order to link an
Objective-C or Objective-C++ program.
-nostartfiles
Do not use the standard system startup files when linking. The
standard system libraries are used normally, unless -nostdlib or
-nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the
libraries you specify will be passed to the linker, options
specifying linkage of the system libraries, such as
"-static-libgcc" or "-shared-libgcc", will be ignored. The
standard startup files are used normally, unless -nostartfiles is
used. The compiler may generate calls to "memcmp", "memset",
"memcpy" and "memmove". These entries are usually resolved by
entries in libc. These entry points should be supplied through
some other mechanism when this option is specified.
-nostdlib
Do not use the standard system startup files or libraries when
linking. No startup files and only the libraries you specify will
be passed to the linker, options specifying linkage of the system
libraries, such as "-static-libgcc" or "-shared-libgcc", will be
ignored. The compiler may generate calls to "memcmp", "memset",
"memcpy" and "memmove". These entries are usually resolved by
entries in libc. These entry points should be supplied through
some other mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and
-nodefaultlibs is libgcc.a, a library of internal subroutines which
GCC uses to overcome shortcomings of particular machines, or
special needs for some languages.
In most cases, you need libgcc.a even when you want to avoid other
standard libraries. In other words, when you specify -nostdlib or
-nodefaultlibs you should usually specify -lgcc as well. This
ensures that you have no unresolved references to internal GCC
library subroutines. (For example, __main, used to ensure C++
constructors will be called.)
-pie
Produce a position independent executable on targets that support
it. For predictable results, you must also specify the same set of
options that were used to generate code (-fpie, -fPIE, or model
suboptions) when you specify this option.
-rdynamic
Pass the flag -export-dynamic to the ELF linker, on targets that
support it. This instructs the linker to add all symbols, not only
used ones, to the dynamic symbol table. This option is needed for
some uses of "dlopen" or to allow obtaining backtraces from within
a program.
-s Remove all symbol table and relocation information from the
executable.
-static
On systems that support dynamic linking, this prevents linking with
the shared libraries. On other systems, this option has no effect.
-shared
Produce a shared object which can then be linked with other objects
to form an executable. Not all systems support this option. For
predictable results, you must also specify the same set of options
that were used to generate code (-fpic, -fPIC, or model suboptions)
when you specify this option.[1]
-shared-libgcc-static-libgcc
On systems that provide libgcc as a shared library, these options
force the use of either the shared or static version respectively.
If no shared version of libgcc was built when the compiler was
configured, these options have no effect.
There are several situations in which an application should use the
shared libgcc instead of the static version. The most common of
these is when the application wishes to throw and catch exceptions
across different shared libraries. In that case, each of the
libraries as well as the application itself should use the shared
libgcc.
Therefore, the G++ and GCJ drivers automatically add -shared-libgcc
whenever you build a shared library or a main executable, because
C++ and Java programs typically use exceptions, so this is the
right thing to do.
If, instead, you use the GCC driver to create shared libraries, you
may find that they will not always be linked with the shared
libgcc. If GCC finds, at its configuration time, that you have a
non-GNU linker or a GNU linker that does not support option
--eh-frame-hdr, it will link the shared version of libgcc into
shared libraries by default. Otherwise, it will take advantage of
the linker and optimize away the linking with the shared version of
libgcc, linking with the static version of libgcc by default. This
allows exceptions to propagate through such shared libraries,
without incurring relocation costs at library load time.
However, if a library or main executable is supposed to throw or
catch exceptions, you must link it using the G++ or GCJ driver, as
appropriate for the languages used in the program, or using the
option -shared-libgcc, such that it is linked with the shared
libgcc.
-static-libstdc++
When the g++ program is used to link a C++ program, it will
normally automatically link against libstdc++. If libstdc++ is
available as a shared library, and the -static option is not used,
then this will link against the shared version of libstdc++. That
is normally fine. However, it is sometimes useful to freeze the
version of libstdc++ used by the program without going all the way
to a fully static link. The -static-libstdc++ option directs the
g++ driver to link libstdc++ statically, without necessarily
linking other libraries statically.
-symbolic
Bind references to global symbols when building a shared object.
Warn about any unresolved references (unless overridden by the link
editor option -Xlinker-z-Xlinkerdefs). Only a few systems
support this option.
-Tscript
Use script as the linker script. This option is supported by most
systems using the GNU linker. On some targets, such as bare-board
targets without an operating system, the -T option may be required
when linking to avoid references to undefined symbols.
-Xlinkeroption
Pass option as an option to the linker. You can use this to supply
system-specific linker options that GCC does not recognize.
If you want to pass an option that takes a separate argument, you
must use -Xlinker twice, once for the option and once for the
argument. For example, to pass -assertdefinitions, you must write
-Xlinker-assert-Xlinkerdefinitions. It does not work to write
-Xlinker"-assertdefinitions", because this passes the entire
string as a single argument, which is not what the linker expects.
When using the GNU linker, it is usually more convenient to pass
arguments to linker options using the option=value syntax than as
separate arguments. For example, you can specify -Xlinker-Map=output.map rather than -Xlinker-Map-Xlinkeroutput.map.
Other linkers may not support this syntax for command-line options.
-Wl,option
Pass option as an option to the linker. If option contains commas,
it is split into multiple options at the commas. You can use this
syntax to pass an argument to the option. For example,
-Wl,-Map,output.map passes -Mapoutput.map to the linker. When
using the GNU linker, you can also get the same effect with
-Wl,-Map=output.map.
-usymbol
Pretend the symbol symbol is undefined, to force linking of library
modules to define it. You can use -u multiple times with different
symbols to force loading of additional library modules.
OptionsforDirectorySearch
These options specify directories to search for header files, for
libraries and for parts of the compiler:
-Idir
Add the directory dir to the head of the list of directories to be
searched for header files. This can be used to override a system
header file, substituting your own version, since these directories
are searched before the system header file directories. However,
you should not use this option to add directories that contain
vendor-supplied system header files (use -isystem for that). If
you use more than one -I option, the directories are scanned in
left-to-right order; the standard system directories come after.
If a standard system include directory, or a directory specified
with -isystem, is also specified with -I, the -I option will be
ignored. The directory will still be searched but as a system
directory at its normal position in the system include chain. This
is to ensure that GCC's procedure to fix buggy system headers and
the ordering for the include_next directive are not inadvertently
changed. If you really need to change the search order for system
directories, use the -nostdinc and/or -isystem options.
-iplugindir=dir
Set the directory to search for plugins that are passed by
-fplugin=name instead of -fplugin=path/name.so. This option is not
meant to be used by the user, but only passed by the driver.
-iquotedir
Add the directory dir to the head of the list of directories to be
searched for header files only for the case of #include"file";
they are not searched for #include<file>, otherwise just like -I.
-Ldir
Add directory dir to the list of directories to be searched for -l.
-Bprefix
This option specifies where to find the executables, libraries,
include files, and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms
cpp, cc1, as and ld. It tries prefix as a prefix for each program
it tries to run, both with and without machine/version/.
For each subprogram to be run, the compiler driver first tries the
-B prefix, if any. If that name is not found, or if -B was not
specified, the driver tries two standard prefixes, /usr/lib/gcc/
and /usr/local/lib/gcc/. If neither of those results in a file
name that is found, the unmodified program name is searched for
using the directories specified in your PATH environment variable.
The compiler will check to see if the path provided by the -B
refers to a directory, and if necessary it will add a directory
separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to
libraries in the linker, because the compiler translates these
options into -L options for the linker. They also apply to
includes files in the preprocessor, because the compiler translates
these options into -isystem options for the preprocessor. In this
case, the compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched for using
the -B prefix, if needed. If it is not found there, the two
standard prefixes above are tried, and that is all. The file is
left out of the link if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use
the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/,
where N is a number in the range 0 to 9, then it will be replaced
by [dir/]include. This is to help with boot-strapping the
compiler.
-specs=file
Process file after the compiler reads in the standard specs file,
in order to override the defaults which the gcc driver program uses
when determining what switches to pass to cc1, cc1plus, as, ld,
etc. More than one -specs=file can be specified on the command
line, and they are processed in order, from left to right.
--sysroot=dir
Use dir as the logical root directory for headers and libraries.
For example, if the compiler would normally search for headers in
/usr/include and libraries in /usr/lib, it will instead search
dir/usr/include and dir/usr/lib.
If you use both this option and the -isysroot option, then the
--sysroot option will apply to libraries, but the -isysroot option
will apply to header files.
The GNU linker (beginning with version 2.16) has the necessary
support for this option. If your linker does not support this
option, the header file aspect of --sysroot will still work, but
the library aspect will not.
-I- This option has been deprecated. Please use -iquote instead for -I
directories before the -I- and remove the -I-. Any directories you
specify with -I options before the -I- option are searched only for
the case of #include"file"; they are not searched for #include<file>.
If additional directories are specified with -I options after the
-I-, these directories are searched for all #include directives.
(Ordinarily all-I directories are used this way.)
In addition, the -I- option inhibits the use of the current
directory (where the current input file came from) as the first
search directory for #include"file". There is no way to override
this effect of -I-. With -I. you can specify searching the
directory that was current when the compiler was invoked. That is
not exactly the same as what the preprocessor does by default, but
it is often satisfactory.
-I- does not inhibit the use of the standard system directories for
header files. Thus, -I- and -nostdinc are independent.
SpecifyingTargetMachineandCompilerVersion
The usual way to run GCC is to run the executable called gcc, or
machine-gcc when cross-compiling, or machine-gcc-version to run a
version other than the one that was installed last.
HardwareModelsandConfigurations
Each target machine types can have its own special options, starting
with -m, to choose among various hardware models or
configurations---for example, 68010 vs 68020, floating coprocessor or
none. A single installed version of the compiler can compile for any
model or configuration, according to the options specified.
Some configurations of the compiler also support additional special
options, usually for compatibility with other compilers on the same
platform.
AdaptevaEpiphanyOptions
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file
Don't allocate any register in the range "r32"..."r63". That
allows code to run on hardware variants that lack these registers.
-mprefer-short-insn-regs
Preferrentially allocate registers that allow short instruction
generation. This can result in increasesd instruction count, so if
this reduces or increases code size might vary from case to case.
-mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions.
This cost is only a heuristic and is not guaranteed to produce
consistent results across releases.
-mcmove
Enable the generation of conditional moves.
-mnops=num
Emit num nops before every other generated instruction.
-mno-soft-cmpsf
For single-precision floating-point comparisons, emit an fsub
instruction and test the flags. This is faster than a software
comparison, but can get incorrect results in the presence of NaNs,
or when two different small numbers are compared such that their
difference is calculated as zero. The default is -msoft-cmpsf,
which uses slower, but IEEE-compliant, software comparisons.
-mstack-offset=num
Set the offset between the top of the stack and the stack pointer.
E.g., a value of 8 means that the eight bytes in the range
sp+0...sp+7 can be used by leaf functions without stack allocation.
Values other than 8 or 16 are untested and unlikely to work. Note
also that this option changes the ABI, compiling a program with a
different stack offset than the libraries have been compiled with
will generally not work. This option can be useful if you want to
evaluate if a different stack offset would give you better code,
but to actually use a different stack offset to build working
programs, it is recommended to configure the toolchain with the
appropriate --with-stack-offset=num option.
-mno-round-nearest
Make the scheduler assume that the rounding mode has been set to
truncating. The default is -mround-nearest.
-mlong-calls
If not otherwise specified by an attribute, assume all calls might
be beyond the offset range of the b / bl instructions, and
therefore load the function address into a register before
performing a (otherwise direct) call. This is the default.
-mshort-calls
If not otherwise specified by an attribute, assume all direct calls
are in the range of the b / bl instructions, so use these
instructions for direct calls. The default is -mlong-calls.
-msmall16
Assume addresses can be loaded as 16-bit unsigned values. This
does not apply to function addresses for which -mlong-calls
semantics are in effect.
-mfp-mode=mode
Set the prevailing mode of the floating-point unit. This
determines the floating-point mode that is provided and expected at
function call and return time. Making this mode match the mode you
predominantly need at function start can make your programs smaller
and faster by avoiding unnecessary mode switches.
mode can be set to one the following values:
caller
Any mode at function entry is valid, and retained or restored
when the function returns, and when it calls other functions.
This mode is useful for compiling libraries or other
compilation units you might want to incorporate into different
programs with different prevailing FPU modes, and the
convenience of being able to use a single object file outweighs
the size and speed overhead for any extra mode switching that
might be needed, compared with what would be needed with a more
specific choice of prevailing FPU mode.
truncate
This is the mode used for floating-point calculations with
truncating (i.e. round towards zero) rounding mode. That
includes conversion from floating point to integer.
round-nearest
This is the mode used for floating-point calculations with
round-to-nearest-or-even rounding mode.
int This is the mode used to perform integer calculations in the
FPU, e.g. integer multiply, or integer multiply-and-
accumulate.
The default is -mfp-mode=caller-mnosplit-lohi-mno-postinc-mno-postmodify
Code generation tweaks that disable, respectively, splitting of
32-bit loads, generation of post-increment addresses, and
generation of post-modify addresses. The defaults are msplit-lohi,
-mpost-inc, and -mpost-modify.
-mnovect-double
Change the preferred SIMD mode to SImode. The default is
-mvect-double, which uses DImode as preferred SIMD mode.
-max-vect-align=num
The maximum alignment for SIMD vector mode types. num may be 4 or
8. The default is 8. Note that this is an ABI change, even though
many library function interfaces will be unaffected, if they don't
use SIMD vector modes in places where they affect size and/or
alignment of relevant types.
-msplit-vecmove-early
Split vector moves into single word moves before reload. In theory
this could give better register allocation, but so far the reverse
seems to be generally the case.
-m1reg-reg
Specify a register to hold the constant -1, which makes loading
small negative constants and certain bitmasks faster. Allowable
values for reg are r43 and r63, which specify to use that register
as a fixed register, and none, which means that no register is used
for this purpose. The default is -m1reg-none.
ARMOptions
These -m options are defined for Advanced RISC Machines (ARM)
architectures:
-mabi=name
Generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure
Call Standard for all functions, even if this is not strictly
necessary for correct execution of the code. Specifying
-fomit-frame-pointer with this option will cause the stack frames
not to be generated for leaf functions. The default is
-mno-apcs-frame.
-mapcs
This is a synonym for -mapcs-frame.
-mthumb-interwork
Generate code that supports calling between the ARM and Thumb
instruction sets. Without this option, on pre-v5 architectures,
the two instruction sets cannot be reliably used inside one
program. The default is -mno-thumb-interwork, since slightly
larger code is generated when -mthumb-interwork is specified. In
AAPCS configurations this option is meaningless.
-mno-sched-prolog
Prevent the reordering of instructions in the function prologue, or
the merging of those instruction with the instructions in the
function's body. This means that all functions will start with a
recognizable set of instructions (or in fact one of a choice from a
small set of different function prologues), and this information
can be used to locate the start if functions inside an executable
piece of code. The default is -msched-prolog.
-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible values are:
soft, softfp and hard.
Specifying soft causes GCC to generate output containing library
calls for floating-point operations. softfp allows the generation
of code using hardware floating-point instructions, but still uses
the soft-float calling conventions. hard allows generation of
floating-point instructions and uses FPU-specific calling
conventions.
The default depends on the specific target configuration. Note
that the hard-float and soft-float ABIs are not link-compatible;
you must compile your entire program with the same ABI, and link
with a compatible set of libraries.
-mlittle-endian
Generate code for a processor running in little-endian mode. This
is the default for all standard configurations.
-mbig-endian
Generate code for a processor running in big-endian mode; the
default is to compile code for a little-endian processor.
-mwords-little-endian
This option only applies when generating code for big-endian
processors. Generate code for a little-endian word order but a
big-endian byte order. That is, a byte order of the form 32107654.
Note: this option should only be used if you require compatibility
with code for big-endian ARM processors generated by versions of
the compiler prior to 2.8. This option is now deprecated.
-march=name
This specifies the name of the target ARM architecture. GCC uses
this name to determine what kind of instructions it can emit when
generating assembly code. This option can be used in conjunction
with or instead of the -mcpu= option. Permissible names are:
armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5, armv5t, armv5e,
armv5te, armv6, armv6j, armv6t2, armv6z, armv6zk, armv6-m, armv7,
armv7-a, armv7-r, armv7-m, armv7e-m, iwmmxt, iwmmxt2, ep9312.
-march=native causes the compiler to auto-detect the architecture
of the build computer. At present, this feature is only supported
on Linux, and not all architectures are recognized. If the auto-
detect is unsuccessful the option has no effect.
-mtune=name
This option specifies the name of the target ARM processor for
which GCC should tune the performance of the code. For some ARM
implementations better performance can be obtained by using this
option. Permissible names are: arm2, arm250, arm3, arm6, arm60,
arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm, arm7di,
arm7dmi, arm70, arm700, arm700i, arm710, arm710c, arm7100, arm720,
arm7500, arm7500fe, arm7tdmi, arm7tdmi-s, arm710t, arm720t,
arm740t, strongarm, strongarm110, strongarm1100, strongarm1110,
arm8, arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s,
arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi, arm10tdmi,
arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s,
arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s, arm1156t2f-s,
arm1176jz-s, arm1176jzf-s, cortex-a5, cortex-a7, cortex-a8,
cortex-a9, cortex-a15, cortex-r4, cortex-r4f, cortex-r5, cortex-m4,
cortex-m3, cortex-m1, cortex-m0, xscale, iwmmxt, iwmmxt2, ep9312,
fa526, fa626, fa606te, fa626te, fmp626, fa726te.
-mtune=generic-arch specifies that GCC should tune the performance
for a blend of processors within architecture arch. The aim is to
generate code that run well on the current most popular processors,
balancing between optimizations that benefit some CPUs in the
range, and avoiding performance pitfalls of other CPUs. The
effects of this option may change in future GCC versions as CPU
models come and go.
-mtune=native causes the compiler to auto-detect the CPU of the
build computer. At present, this feature is only supported on
Linux, and not all architectures are recognized. If the auto-
detect is unsuccessful the option has no effect.
-mcpu=name
This specifies the name of the target ARM processor. GCC uses this
name to derive the name of the target ARM architecture (as if
specified by -march) and the ARM processor type for which to tune
for performance (as if specified by -mtune). Where this option is
used in conjunction with -march or -mtune, those options take
precedence over the appropriate part of this option.
Permissible names for this option are the same as those for -mtune.
-mcpu=generic-arch is also permissible, and is equivalent to
-march=arch-mtune=generic-arch. See -mtune for more information.
-mcpu=native causes the compiler to auto-detect the CPU of the
build computer. At present, this feature is only supported on
Linux, and not all architectures are recognized. If the auto-
detect is unsuccessful the option has no effect.
-mfpu=name-mfpe=number-mfp=number
This specifies what floating-point hardware (or hardware emulation)
is available on the target. Permissible names are: fpa, fpe2,
fpe3, maverick, vfp, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16,
vfpv3xd, vfpv3xd-fp16, neon, neon-fp16, vfpv4, vfpv4-d16,
fpv4-sp-d16 and neon-vfpv4. -mfp and -mfpe are synonyms for
-mfpu=fpenumber, for compatibility with older versions of GCC.
If -msoft-float is specified this specifies the format of floating-
point values.
If the selected floating-point hardware includes the NEON extension
(e.g. -mfpu=neon), note that floating-point operations will not be
used by GCC's auto-vectorization pass unless
-funsafe-math-optimizations is also specified. This is because
NEON hardware does not fully implement the IEEE 754 standard for
floating-point arithmetic (in particular denormal values are
treated as zero), so the use of NEON instructions may lead to a
loss of precision.
-mfp16-format=name
Specify the format of the "__fp16" half-precision floating-point
type. Permissible names are none, ieee, and alternative; the
default is none, in which case the "__fp16" type is not defined.
-mstructure-size-boundary=n
The size of all structures and unions will be rounded up to a
multiple of the number of bits set by this option. Permissible
values are 8, 32 and 64. The default value varies for different
toolchains. For the COFF targeted toolchain the default value is
8. A value of 64 is only allowed if the underlying ABI supports
it.
Specifying the larger number can produce faster, more efficient
code, but can also increase the size of the program. Different
values are potentially incompatible. Code compiled with one value
cannot necessarily expect to work with code or libraries compiled
with another value, if they exchange information using structures
or unions.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a "noreturn"
function. It will be executed if the function tries to return.
-mlong-calls-mno-long-calls
Tells the compiler to perform function calls by first loading the
address of the function into a register and then performing a
subroutine call on this register. This switch is needed if the
target function will lie outside of the 64 megabyte addressing
range of the offset based version of subroutine call instruction.
Even if this switch is enabled, not all function calls will be
turned into long calls. The heuristic is that static functions,
functions that have the short-call attribute, functions that are
inside the scope of a #pragmano_long_calls directive and functions
whose definitions have already been compiled within the current
compilation unit, will not be turned into long calls. The
exception to this rule is that weak function definitions, functions
with the long-call attribute or the section attribute, and
functions that are within the scope of a #pragmalong_calls
directive, will always be turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls
will restore the default behavior, as will placing the function
calls within the scope of a #pragmalong_calls_off directive. Note
these switches have no effect on how the compiler generates code to
handle function calls via function pointers.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather
than loading it in the prologue for each function. The runtime
system is responsible for initializing this register with an
appropriate value before execution begins.
-mpic-register=reg
Specify the register to be used for PIC addressing. The default is
R10 unless stack-checking is enabled, when R9 is used.
-mcirrus-fix-invalid-insns
Insert NOPs into the instruction stream to in order to work around
problems with invalid Maverick instruction combinations. This
option is only valid if the -mcpu=ep9312 option has been used to
enable generation of instructions for the Cirrus Maverick floating-
point co-processor. This option is not enabled by default, since
the problem is only present in older Maverick implementations. The
default can be re-enabled by use of the
-mno-cirrus-fix-invalid-insns switch.
-mpoke-function-name
Write the name of each function into the text section, directly
preceding the function prologue. The generated code is similar to
this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of
"pc" stored at "fp + 0". If the trace function then looks at
location "pc - 12" and the top 8 bits are set, then we know that
there is a function name embedded immediately preceding this
location and has length "((pc[-3]) & 0xff000000)".
-mthumb-marm
Select between generating code that executes in ARM and Thumb
states. The default for most configurations is to generate code
that executes in ARM state, but the default can be changed by
configuring GCC with the --with-mode=state configure option.
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb Procedure
Call Standard for all non-leaf functions. (A leaf function is one
that does not call any other functions.) The default is
-mno-tpcs-frame.
-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb Procedure
Call Standard for all leaf functions. (A leaf function is one that
does not call any other functions.) The default is
-mno-apcs-leaf-frame.
-mcallee-super-interworking
Gives all externally visible functions in the file being compiled
an ARM instruction set header which switches to Thumb mode before
executing the rest of the function. This allows these functions to
be called from non-interworking code. This option is not valid in
AAPCS configurations because interworking is enabled by default.
-mcaller-super-interworking
Allows calls via function pointers (including virtual functions) to
execute correctly regardless of whether the target code has been
compiled for interworking or not. There is a small overhead in the
cost of executing a function pointer if this option is enabled.
This option is not valid in AAPCS configurations because
interworking is enabled by default.
-mtp=name
Specify the access model for the thread local storage pointer. The
valid models are soft, which generates calls to "__aeabi_read_tp",
cp15, which fetches the thread pointer from "cp15" directly
(supported in the arm6k architecture), and auto, which uses the
best available method for the selected processor. The default
setting is auto.
-mtls-dialect=dialect
Specify the dialect to use for accessing thread local storage. Two
dialects are supported --- gnu and gnu2. The gnu dialect selects
the original GNU scheme for supporting local and global dynamic TLS
models. The gnu2 dialect selects the GNU descriptor scheme, which
provides better performance for shared libraries. The GNU
descriptor scheme is compatible with the original scheme, but does
require new assembler, linker and library support. Initial and
local exec TLS models are unaffected by this option and always use
the original scheme.
-mword-relocations
Only generate absolute relocations on word-sized values (i.e.
R_ARM_ABS32). This is enabled by default on targets (uClinux,
SymbianOS) where the runtime loader imposes this restriction, and
when -fpic or -fPIC is specified.
-mfix-cortex-m3-ldrd
Some Cortex-M3 cores can cause data corruption when "ldrd"
instructions with overlapping destination and base registers are
used. This option avoids generating these instructions. This
option is enabled by default when -mcpu=cortex-m3 is specified.
-munaligned-access-mno-unaligned-access
Enables (or disables) reading and writing of 16- and 32- bit values
from addresses that are not 16- or 32- bit aligned. By default
unaligned access is disabled for all pre-ARMv6 and all ARMv6-M
architectures, and enabled for all other architectures. If
unaligned access is not enabled then words in packed data
structures will be accessed a byte at a time.
The ARM attribute "Tag_CPU_unaligned_access" will be set in the
generated object file to either true or false, depending upon the
setting of this option. If unaligned access is enabled then the
preprocessor symbol "__ARM_FEATURE_UNALIGNED" will also be defined.
AVROptions-mmcu=mcu
Specify Atmel AVR instruction set architectures (ISA) or MCU type.
The default for this option is@tie{}"avr2".
GCC supports the following AVR devices and ISAs:
"avr2"
"Classic" devices with up to 8@tie{}KiB of program memory.
mcu@tie{}= "attiny22", "attiny26", "at90c8534", "at90s2313",
"at90s2323", "at90s2333", "at90s2343", "at90s4414",
"at90s4433", "at90s4434", "at90s8515", "at90s8535".
"avr25"
"Classic" devices with up to 8@tie{}KiB of program memory and
with the "MOVW" instruction. mcu@tie{}= "ata6289", "attiny13",
"attiny13a", "attiny2313", "attiny2313a", "attiny24",
"attiny24a", "attiny25", "attiny261", "attiny261a",
"attiny43u", "attiny4313", "attiny44", "attiny44a", "attiny45",
"attiny461", "attiny461a", "attiny48", "attiny84", "attiny84a",
"attiny85", "attiny861", "attiny861a", "attiny87", "attiny88",
"at86rf401".
"avr3"
"Classic" devices with 16@tie{}KiB up to 64@tie{}KiB of
program memory. mcu@tie{}= "at43usb355", "at76c711".
"avr31"
"Classic" devices with 128@tie{}KiB of program memory.
mcu@tie{}= "atmega103", "at43usb320".
"avr35"
"Classic" devices with 16@tie{}KiB up to 64@tie{}KiB of program
memory and with the "MOVW" instruction. mcu@tie{}=
"atmega16u2", "atmega32u2", "atmega8u2", "attiny167",
"at90usb162", "at90usb82".
"avr4"
"Enhanced" devices with up to 8@tie{}KiB of program memory.
mcu@tie{}= "atmega48", "atmega48a", "atmega48p", "atmega8",
"atmega8hva", "atmega8515", "atmega8535", "atmega88",
"atmega88a", "atmega88p", "atmega88pa", "at90pwm1", "at90pwm2",
"at90pwm2b", "at90pwm3", "at90pwm3b", "at90pwm81".
"avr5"
"Enhanced" devices with 16@tie{}KiB up to 64@tie{}KiB of
program memory. mcu@tie{}= "atmega16", "atmega16a",
"atmega16hva", "atmega16hva2", "atmega16hvb", "atmega16m1",
"atmega16u4", "atmega161", "atmega162", "atmega163",
"atmega164a", "atmega164p", "atmega165", "atmega165a",
"atmega165p", "atmega168", "atmega168a", "atmega168p",
"atmega169", "atmega169a", "atmega169p", "atmega169pa",
"atmega32", "atmega32c1", "atmega32hvb", "atmega32m1",
"atmega32u4", "atmega32u6", "atmega323", "atmega324a",
"atmega324p", "atmega324pa", "atmega325", "atmega325a",
"atmega325p", "atmega3250", "atmega3250a", "atmega3250p",
"atmega328", "atmega328p", "atmega329", "atmega329a",
"atmega329p", "atmega329pa", "atmega3290", "atmega3290a",
"atmega3290p", "atmega406", "atmega64", "atmega64c1",
"atmega64hve", "atmega64m1", "atmega640", "atmega644",
"atmega644a", "atmega644p", "atmega644pa", "atmega645",
"atmega645a", "atmega645p", "atmega6450", "atmega6450a",
"atmega6450p", "atmega649", "atmega649a", "atmega649p",
"atmega6490", "at90can32", "at90can64", "at90pwm216",
"at90pwm316", "at90scr100", "at90usb646", "at90usb647",
"at94k", "m3000".
"avr51"
"Enhanced" devices with 128@tie{}KiB of program memory.
mcu@tie{}= "atmega128", "atmega128rfa1", "atmega1280",
"atmega1281", "atmega1284p", "at90can128", "at90usb1286",
"at90usb1287".
"avr6"
"Enhanced" devices with 3-byte PC, i.e. with more than
128@tie{}KiB of program memory. mcu@tie{}= "atmega2560",
"atmega2561".
"avrxmega2"
"XMEGA" devices with more than 8@tie{}KiB and up to 64@tie{}KiB
of program memory. mcu@tie{}= "atxmega16a4", "atxmega16d4",
"atxmega16x1", "atxmega32a4", "atxmega32d4", "atxmega32x1".
"avrxmega4"
"XMEGA" devices with more than 64@tie{}KiB and up to
128@tie{}KiB of program memory. mcu@tie{}= "atxmega64a3",
"atxmega64d3".
"avrxmega5"
"XMEGA" devices with more than 64@tie{}KiB and up to
128@tie{}KiB of program memory and more than 64@tie{}KiB of
RAM. mcu@tie{}= "atxmega64a1", "atxmega64a1u".
"avrxmega6"
"XMEGA" devices with more than 128@tie{}KiB of program memory.
mcu@tie{}= "atxmega128a3", "atxmega128d3", "atxmega192a3",
"atxmega192d3", "atxmega256a3", "atxmega256a3b",
"atxmega256a3bu", "atxmega256d3".
"avrxmega7"
"XMEGA" devices with more than 128@tie{}KiB of program memory
and more than 64@tie{}KiB of RAM. mcu@tie{}= "atxmega128a1",
"atxmega128a1u".
"avr1"
This ISA is implemented by the minimal AVR core and supported
for assembler only. mcu@tie{}= "attiny11", "attiny12",
"attiny15", "attiny28", "at90s1200".
-maccumulate-args
Accumulate outgoing function arguments and acquire/release the
needed stack space for outgoing function arguments once in function
prologue/epilogue. Without this option, outgoing arguments are
pushed before calling a function and popped afterwards.
Popping the arguments after the function call can be expensive on
AVR so that accumulating the stack space might lead to smaller
executables because arguments need not to be removed from the stack
after such a function call.
This option can lead to reduced code size for functions that
perform several calls to functions that get their arguments on the
stack like calls to printf-like functions.
-mbranch-cost=cost
Set the branch costs for conditional branch instructions to cost.
Reasonable values for cost are small, non-negative integers. The
default branch cost is 0.
-mcall-prologues
Functions prologues/epilogues are expanded as calls to appropriate
subroutines. Code size is smaller.
-mint8
Assume "int" to be 8-bit integer. This affects the sizes of all
types: a "char" is 1 byte, an "int" is 1 byte, a "long" is 2 bytes,
and "long long" is 4 bytes. Please note that this option does not
conform to the C standards, but it results in smaller code size.
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code
size is smaller.
-mrelax
Try to replace "CALL" resp. "JMP" instruction by the shorter
"RCALL" resp. "RJMP" instruction if applicable. Setting "-mrelax"
just adds the "--relax" option to the linker command line when the
linker is called.
Jump relaxing is performed by the linker because jump offsets are
not known before code is located. Therefore, the assembler code
generated by the compiler is the same, but the instructions in the
executable may differ from instructions in the assembler code.
Relaxing must be turned on if linker stubs are needed, see the
section on "EIND" and linker stubs below.
-mshort-calls
This option has been deprecated and will be removed in GCC 4.8.
See "-mrelax" for a replacement.
Use "RCALL"/"RJMP" instructions even on devices with 16@tie{}KiB or
more of program memory, i.e. on devices that have the "CALL" and
"JMP" instructions.
-msp8
Treat the stack pointer register as an 8-bit register, i.e. assume
the high byte of the stack pointer is zero. In general, you don't
need to set this option by hand.
This option is used internally by the compiler to select and build
multilibs for architectures "avr2" and "avr25". These
architectures mix devices with and without "SPH". For any setting
other than "-mmcu=avr2" or "-mmcu=avr25" the compiler driver will
add or remove this option from the compiler proper's command line,
because the compiler then knows if the device or architecture has
an 8-bit stack pointer and thus no "SPH" register or not.
-mstrict-X
Use address register "X" in a way proposed by the hardware. This
means that "X" is only used in indirect, post-increment or pre-
decrement addressing.
Without this option, the "X" register may be used in the same way
as "Y" or "Z" which then is emulated by additional instructions.
For example, loading a value with "X+const" addressing with a small
non-negative "const < 64" to a register Rn is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const
-mtiny-stack
Only change the lower 8@tie{}bits of the stack pointer.
"EIND" and Devices with more than 128 Ki Bytes of Flash
Pointers in the implementation are 16@tie{}bits wide. The address of a
function or label is represented as word address so that indirect jumps
and calls can target any code address in the range of 64@tie{}Ki words.
In order to facilitate indirect jump on devices with more than
128@tie{}Ki bytes of program memory space, there is a special function
register called "EIND" that serves as most significant part of the
target address when "EICALL" or "EIJMP" instructions are used.
Indirect jumps and calls on these devices are handled as follows by the
compiler and are subject to some limitations:
o The compiler never sets "EIND".
o The compiler uses "EIND" implicitely in "EICALL"/"EIJMP"
instructions or might read "EIND" directly in order to emulate an
indirect call/jump by means of a "RET" instruction.
o The compiler assumes that "EIND" never changes during the startup
code or during the application. In particular, "EIND" is not
saved/restored in function or interrupt service routine
prologue/epilogue.
o For indirect calls to functions and computed goto, the linker
generates stubs. Stubs are jump pads sometimes also called
trampolines. Thus, the indirect call/jump jumps to such a stub.
The stub contains a direct jump to the desired address.
o Linker relaxation must be turned on so that the linker will
generate the stubs correctly an all situaltion. See the compiler
option "-mrelax" and the linler option "--relax". There are corner
cases where the linker is supposed to generate stubs but aborts
without relaxation and without a helpful error message.
o The default linker script is arranged for code with "EIND = 0". If
code is supposed to work for a setup with "EIND != 0", a custom
linker script has to be used in order to place the sections whose
name start with ".trampolines" into the segment where "EIND" points
to.
o The startup code from libgcc never sets "EIND". Notice that
startup code is a blend of code from libgcc and AVR-LibC. For the
impact of AVR-LibC on "EIND", see the AVR-LibC user manual
("http://nongnu.org/avr-libc/user-manual/").
o It is legitimate for user-specific startup code to set up "EIND"
early, for example by means of initialization code located in
section ".init3". Such code runs prior to general startup code that
initializes RAM and calls constructors, but after the bit of
startup code from AVR-LibC that sets "EIND" to the segment where
the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
The "__trampolines_start" symbol is defined in the linker script.
o Stubs are generated automatically by the linker if the following
two conditions are met:
-<The address of a label is taken by means of the "gs" modifier>
(short for generatestubs) like so:
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))
-<The final location of that label is in a code segment>
outside the segment where the stubs are located.
o The compiler emits such "gs" modifiers for code labels in the
following situations:
-<Taking address of a function or code label.>
-<Computed goto.>
-<If prologue-save function is used, see -mcall-prologues>
command-line option.
-<Switch/case dispatch tables. If you do not want such dispatch>
tables you can specify the -fno-jump-tables command-line
option.
-<C and C++ constructors/destructors called during
startup/shutdown.>
-<If the tools hit a "gs()" modifier explained above.>
o Jumping to non-symbolic addresses like so is not supported:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}
Instead, a stub has to be set up, i.e. the function has to be
called through a symbol ("func_4" in the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
and the application be linked with "-Wl,--defsym,func_4=0x4".
Alternatively, "func_4" can be defined in the linker script.
Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function
Registers
Some AVR devices support memories larger than the 64@tie{}KiB range
that can be accessed with 16-bit pointers. To access memory locations
outside this 64@tie{}KiB range, the contentent of a "RAMP" register is
used as high part of the address: The "X", "Y", "Z" address register is
concatenated with the "RAMPX", "RAMPY", "RAMPZ" special function
register, respectively, to get a wide address. Similarly, "RAMPD" is
used together with direct addressing.
o The startup code initializes the "RAMP" special function registers
with zero.
o If a AVRNamedAddressSpaces,namedaddressspace other than
generic or "__flash" is used, then "RAMPZ" is set as needed before
the operation.
o If the device supports RAM larger than 64@tie{KiB} and the compiler
needs to change "RAMPZ" to accomplish an operation, "RAMPZ" is
reset to zero after the operation.
o If the device comes with a specific "RAMP" register, the ISR
prologue/epilogue saves/restores that SFR and initializes it with
zero in case the ISR code might (implicitly) use it.
o RAM larger than 64@tie{KiB} is not supported by GCC for AVR
targets. If you use inline assembler to read from locations
outside the 16-bit address range and change one of the "RAMP"
registers, you must reset it to zero after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code can test for
the presence or absence of features. Almost any of the following
built-in macros are deduced from device capabilities and thus triggered
by the "-mmcu=" command-line option.
For even more AVR-specific built-in macros see AVRNamedAddressSpaces
and AVRBuilt-inFunctions.
"__AVR_ARCH__"
Build-in macro that resolves to a decimal number that identifies
the architecture and depends on the "-mmcu=mcu" option. Possible
values are:
2, 25, 3, 31, 35, 4, 5, 51, 6, 102, 104, 105, 106, 107
for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5",
"avr51", "avr6", "avrxmega2", "avrxmega4", "avrxmega5",
"avrxmega6", "avrxmega7", respectively. If mcu specifies a device,
this built-in macro is set accordingly. For example, with
"-mmcu=atmega8" the macro will be defined to 4.
"__AVR_Device__"
Setting "-mmcu=device" defines this built-in macro which reflects
the device's name. For example, "-mmcu=atmega8" defines the built-
in macro "__AVR_ATmega8__", "-mmcu=attiny261a" defines
"__AVR_ATtiny261A__", etc.
The built-in macros' names follow the scheme "__AVR_Device__" where
Device is the device name as from the AVR user manual. The
difference between Device in the built-in macro and device in
"-mmcu=device" is that the latter is always lowercase.
If device is not a device but only a core architecture like
"avr51", this macro will not be defined.
"__AVR_XMEGA__"
The device/architecture belongs to the XMEGA family of devices.
"__AVR_HAVE_ELPM__"
The device has the the "ELPM" instruction.
"__AVR_HAVE_ELPMX__"
The device has the "ELPM Rn,Z" and "ELPM Rn,Z+" instructions.
"__AVR_HAVE_MOVW__"
The device has the "MOVW" instruction to perform 16-bit register-
register moves.
"__AVR_HAVE_LPMX__"
The device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions.
"__AVR_HAVE_MUL__"
The device has a hardware multiplier.
"__AVR_HAVE_JMP_CALL__"
The device has the "JMP" and "CALL" instructions. This is the case
for devices with at least 16@tie{}KiB of program memory and if
"-mshort-calls" is not set.
"__AVR_HAVE_EIJMP_EICALL__"
"__AVR_3_BYTE_PC__"
The device has the "EIJMP" and "EICALL" instructions. This is the
case for devices with more than 128@tie{}KiB of program memory.
This also means that the program counter (PC) is 3@tie{}bytes wide.
"__AVR_2_BYTE_PC__"
The program counter (PC) is 2@tie{}bytes wide. This is the case for
devices with up to 128@tie{}KiB of program memory.
"__AVR_HAVE_8BIT_SP__"
"__AVR_HAVE_16BIT_SP__"
The stack pointer (SP) register is treated as 8-bit respectively
16-bit register by the compiler. The definition of these macros is
affected by "-mtiny-stack".
"__AVR_HAVE_SPH__"
"__AVR_SP8__"
The device has the SPH (high part of stack pointer) special
function register or has an 8-bit stack pointer, respectively. The
definition of these macros is affected by "-mmcu=" and in the cases
of "-mmcu=avr2" and "-mmcu=avr25" also by "-msp8".
"__AVR_HAVE_RAMPD__"
"__AVR_HAVE_RAMPX__"
"__AVR_HAVE_RAMPY__"
"__AVR_HAVE_RAMPZ__"
The device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special
function register, respectively.
"__NO_INTERRUPTS__"
This macro reflects the "-mno-interrupts" command line option.
"__AVR_ERRATA_SKIP__"
"__AVR_ERRATA_SKIP_JMP_CALL__"
Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit
instructions because of a hardware erratum. Skip instructions are
"SBRS", "SBRC", "SBIS", "SBIC" and "CPSE". The second macro is
only defined if "__AVR_HAVE_JMP_CALL__" is also set.
"__AVR_SFR_OFFSET__=offset"
Instructions that can address I/O special function registers
directly like "IN", "OUT", "SBI", etc. may use a different address
as if addressed by an instruction to access RAM like "LD" or "STS".
This offset depends on the device architecture and has to be
subtracted from the RAM address in order to get the respective
I/O@tie{}address.
"__WITH_AVRLIBC__"
The compiler is configured to be used together with AVR-Libc. See
the "--with-avrlibc" configure option.
BlackfinOptions-mcpu=cpu[-sirevision]
Specifies the name of the target Blackfin processor. Currently,
cpu can be one of bf512, bf514, bf516, bf518, bf522, bf523, bf524,
bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537,
bf538, bf539, bf542, bf544, bf547, bf548, bf549, bf542m, bf544m,
bf547m, bf548m, bf549m, bf561, bf592. The optional sirevision
specifies the silicon revision of the target Blackfin processor.
Any workarounds available for the targeted silicon revision will be
enabled. If sirevision is none, no workarounds are enabled. If
sirevision is any, all workarounds for the targeted processor will
be enabled. The "__SILICON_REVISION__" macro is defined to two
hexadecimal digits representing the major and minor numbers in the
silicon revision. If sirevision is none, the
"__SILICON_REVISION__" is not defined. If sirevision is any, the
"__SILICON_REVISION__" is defined to be 0xffff. If this optional
sirevision is not used, GCC assumes the latest known silicon
revision of the targeted Blackfin processor.
Support for bf561 is incomplete. For bf561, Only the processor
macro is defined. Without this option, bf532 is used as the
processor by default. The corresponding predefined processor
macros for cpu is to be defined. And for bfin-elf toolchain, this
causes the hardware BSP provided by libgloss to be linked in if
-msim is not given.
-msim
Specifies that the program will be run on the simulator. This
causes the simulator BSP provided by libgloss to be linked in.
This option has effect only for bfin-elf toolchain. Certain other
options, such as -mid-shared-library and -mfdpic, imply -msim.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions.
This avoids the instructions to save, set up and restore frame
pointers and makes an extra register available in leaf functions.
The option -fomit-frame-pointer removes the frame pointer for all
functions, which might make debugging harder.
-mspecld-anomaly
When enabled, the compiler will ensure that the generated code does
not contain speculative loads after jump instructions. If this
option is used, "__WORKAROUND_SPECULATIVE_LOADS" is defined.
-mno-specld-anomaly
Don't generate extra code to prevent speculative loads from
occurring.
-mcsync-anomaly
When enabled, the compiler will ensure that the generated code does
not contain CSYNC or SSYNC instructions too soon after conditional
branches. If this option is used, "__WORKAROUND_SPECULATIVE_SYNCS"
is defined.
-mno-csync-anomaly
Don't generate extra code to prevent CSYNC or SSYNC instructions
from occurring too soon after a conditional branch.
-mlow-64k
When enabled, the compiler is free to take advantage of the
knowledge that the entire program fits into the low 64k of memory.
-mno-low-64k
Assume that the program is arbitrarily large. This is the default.
-mstack-check-l1
Do stack checking using information placed into L1 scratchpad
memory by the uClinux kernel.
-mid-shared-library
Generate code that supports shared libraries via the library ID
method. This allows for execute in place and shared libraries in
an environment without virtual memory management. This option
implies -fPIC. With a bfin-elf target, this option implies -msim.
-mno-id-shared-library
Generate code that doesn't assume ID based shared libraries are
being used. This is the default.
-mleaf-id-shared-library
Generate code that supports shared libraries via the library ID
method, but assumes that this library or executable won't link
against any other ID shared libraries. That allows the compiler to
use faster code for jumps and calls.
-mno-leaf-id-shared-library
Do not assume that the code being compiled won't link against any
ID shared libraries. Slower code will be generated for jump and
call insns.
-mshared-library-id=n
Specified the identification number of the ID based shared library
being compiled. Specifying a value of 0 will generate more compact
code, specifying other values will force the allocation of that
number to the current library but is no more space or time
efficient than omitting this option.
-msep-data
Generate code that allows the data segment to be located in a
different area of memory from the text segment. This allows for
execute in place in an environment without virtual memory
management by eliminating relocations against the text section.
-mno-sep-data
Generate code that assumes that the data segment follows the text
segment. This is the default.
-mlong-calls-mno-long-calls
Tells the compiler to perform function calls by first loading the
address of the function into a register and then performing a
subroutine call on this register. This switch is needed if the
target function lies outside of the 24-bit addressing range of the
offset-based version of subroutine call instruction.
This feature is not enabled by default. Specifying -mno-long-calls
will restore the default behavior. Note these switches have no
effect on how the compiler generates code to handle function calls
via function pointers.
-mfast-fp
Link with the fast floating-point library. This library relaxes
some of the IEEE floating-point standard's rules for checking
inputs against Not-a-Number (NAN), in the interest of performance.
-minline-plt
Enable inlining of PLT entries in function calls to functions that
are not known to bind locally. It has no effect without -mfdpic.
-mmulticore
Build standalone application for multicore Blackfin processor.
Proper start files and link scripts will be used to support
multicore. This option defines "__BFIN_MULTICORE". It can only be
used with -mcpu=bf561[-sirevision]. It can be used with -mcorea or
-mcoreb. If it's used without -mcorea or -mcoreb, single
application/dual core programming model is used. In this model, the
main function of Core B should be named as coreb_main. If it's used
with -mcorea or -mcoreb, one application per core programming model
is used. If this option is not used, single core application
programming model is used.
-mcorea
Build standalone application for Core A of BF561 when using one
application per core programming model. Proper start files and link
scripts will be used to support Core A. This option defines
"__BFIN_COREA". It must be used with -mmulticore.
-mcoreb
Build standalone application for Core B of BF561 when using one
application per core programming model. Proper start files and link
scripts will be used to support Core B. This option defines
"__BFIN_COREB". When this option is used, coreb_main should be used
instead of main. It must be used with -mmulticore.
-msdram
Build standalone application for SDRAM. Proper start files and link
scripts will be used to put the application into SDRAM. Loader
should initialize SDRAM before loading the application into SDRAM.
This option defines "__BFIN_SDRAM".
-micplb
Assume that ICPLBs are enabled at run time. This has an effect on
certain anomaly workarounds. For Linux targets, the default is to
assume ICPLBs are enabled; for standalone applications the default
is off.
C6XOptions-march=name
This specifies the name of the target architecture. GCC uses this
name to determine what kind of instructions it can emit when
generating assembly code. Permissible names are: c62x, c64x,
c64x+, c67x, c67x+, c674x.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target. This is the default.
-msim
Choose startup files and linker script suitable for the simulator.
-msdata=default
Put small global and static data in the .neardata section, which is
pointed to by register "B14". Put small uninitialized global and
static data in the .bss section, which is adjacent to the .neardata
section. Put small read-only data into the .rodata section. The
corresponding sections used for large pieces of data are .fardata,
.far and .const.
-msdata=all
Put all data, not just small objets, into the sections reserved for
small data, and use addressing relative to the "B14" register to
access them.
-msdata=none
Make no use of the sections reserved for small data, and use
absolute addresses to access all data. Put all initialized global
and static data in the .fardata section, and all uninitialized data
in the .far section. Put all constant data into the .const
section.
CRISOptions
These options are defined specifically for the CRIS ports.
-march=architecture-type-mcpu=architecture-type
Generate code for the specified architecture. The choices for
architecture-type are v3, v8 and v10 for respectively ETRAX 4,
ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-
linux-gnu, where the default is v10.
-mtune=architecture-type
Tune to architecture-type everything applicable about the generated
code, except for the ABI and the set of available instructions.
The choices for architecture-type are the same as for
-march=architecture-type.
-mmax-stack-frame=n
Warn when the stack frame of a function exceeds n bytes.
-metrax4-metrax100
The options -metrax4 and -metrax100 are synonyms for -march=v3 and
-march=v8 respectively.
-mmul-bug-workaround-mno-mul-bug-workaround
Work around a bug in the "muls" and "mulu" instructions for CPU
models where it applies. This option is active by default.
-mpdebug
Enable CRIS-specific verbose debug-related information in the
assembly code. This option also has the effect to turn off the
#NO_APP formatted-code indicator to the assembler at the beginning
of the assembly file.
-mcc-init
Do not use condition-code results from previous instruction; always
emit compare and test instructions before use of condition codes.
-mno-side-effects
Do not emit instructions with side-effects in addressing modes
other than post-increment.
-mstack-align-mno-stack-align-mdata-align-mno-data-align-mconst-align-mno-const-align
These options (no-options) arranges (eliminate arrangements) for
the stack-frame, individual data and constants to be aligned for
the maximum single data access size for the chosen CPU model. The
default is to arrange for 32-bit alignment. ABI details such as
structure layout are not affected by these options.
-m32-bit-m16-bit-m8-bit
Similar to the stack- data- and const-align options above, these
options arrange for stack-frame, writable data and constants to all
be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit
alignment.
-mno-prologue-epilogue-mprologue-epilogue
With -mno-prologue-epilogue, the normal function prologue and
epilogue which set up the stack frame are omitted and no return
instructions or return sequences are generated in the code. Use
this option only together with visual inspection of the compiled
code: no warnings or errors are generated when call-saved registers
must be saved, or storage for local variable needs to be allocated.
-mno-gotplt-mgotplt
With -fpic and -fPIC, don't generate (do generate) instruction
sequences that load addresses for functions from the PLT part of
the GOT rather than (traditional on other architectures) calls to
the PLT. The default is -mgotplt.
-melf
Legacy no-op option only recognized with the cris-axis-elf and
cris-axis-linux-gnu targets.
-mlinux
Legacy no-op option only recognized with the cris-axis-linux-gnu
target.
-sim
This option, recognized for the cris-axis-elf arranges to link with
input-output functions from a simulator library. Code, initialized
data and zero-initialized data are allocated consecutively.
-sim2
Like -sim, but pass linker options to locate initialized data at
0x40000000 and zero-initialized data at 0x80000000.
CR16Options
These options are defined specifically for the CR16 ports.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by
default.
-mcr16cplus-mcr16c
Generate code for CR16C or CR16C+ architecture. CR16C+ architecture
is default.
-msim
Links the library libsim.a which is in compatible with simulator.
Applicable to elf compiler only.
-mint32
Choose integer type as 32-bit wide.
-mbit-ops
Generates sbit/cbit instructions for bit manipulations.
-mdata-model=model
Choose a data model. The choices for model are near, far or medium.
medium is default. However, far is not valid when -mcr16c option
is chosen as CR16C architecture does not support far data model.
DarwinOptions
These options are defined for all architectures running the Darwin
operating system.
FSF GCC on Darwin does not create "fat" object files; it will create an
object file for the single architecture that it was built to target.
Apple's GCC on Darwin does create "fat" files if multiple -arch options
are used; it does so by running the compiler or linker multiple times
and joining the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is
determined by the flags that specify the ISA that GCC is targetting,
like -mcpu or -march. The -force_cpusubtype_ALL option can be used to
override this.
The Darwin tools vary in their behavior when presented with an ISA
mismatch. The assembler, as, will only permit instructions to be used
that are valid for the subtype of the file it is generating, so you
cannot put 64-bit instructions in a ppc750 object file. The linker for
shared libraries, /usr/bin/libtool, will fail and print an error if
asked to create a shared library with a less restrictive subtype than
its input files (for instance, trying to put a ppc970 object file in a
ppc7400 library). The linker for executables, ld, will quietly give
the executable the most restrictive subtype of any of its input files.
-Fdir
Add the framework directory dir to the head of the list of
directories to be searched for header files. These directories are
interleaved with those specified by -I options and are scanned in a
left-to-right order.
A framework directory is a directory with frameworks in it. A
framework is a directory with a "Headers" and/or "PrivateHeaders"
directory contained directly in it that ends in ".framework". The
name of a framework is the name of this directory excluding the
".framework". Headers associated with the framework are found in
one of those two directories, with "Headers" being searched first.
A subframework is a framework directory that is in a framework's
"Frameworks" directory. Includes of subframework headers can only
appear in a header of a framework that contains the subframework,
or in a sibling subframework header. Two subframeworks are
siblings if they occur in the same framework. A subframework
should not have the same name as a framework, a warning will be
issued if this is violated. Currently a subframework cannot have
subframeworks, in the future, the mechanism may be extended to
support this. The standard frameworks can be found in
"/System/Library/Frameworks" and "/Library/Frameworks". An example
include looks like "#include <Framework/header.h>", where Framework
denotes the name of the framework and header.h is found in the
"PrivateHeaders" or "Headers" directory.
-iframeworkdir
Like -F except the directory is a treated as a system directory.
The main difference between this -iframework and -F is that with
-iframework the compiler does not warn about constructs contained
within header files found via dir. This option is valid only for
the C family of languages.
-gused
Emit debugging information for symbols that are used. For STABS
debugging format, this enables -feliminate-unused-debug-symbols.
This is by default ON.
-gfull
Emit debugging information for all symbols and types.
-mmacosx-version-min=version
The earliest version of MacOS X that this executable will run on is
version. Typical values of version include 10.1, 10.2, and 10.3.9.
If the compiler was built to use the system's headers by default,
then the default for this option is the system version on which the
compiler is running, otherwise the default is to make choices that
are compatible with as many systems and code bases as possible.
-mkernel
Enable kernel development mode. The -mkernel option sets -static,
-fno-common, -fno-cxa-atexit, -fno-exceptions,
-fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti
where applicable. This mode also sets -mno-altivec, -msoft-float,
-fno-builtin and -mlong-branch for PowerPC targets.
-mone-byte-bool
Override the defaults for bool so that sizeof(bool)==1. By default
sizeof(bool) is 4 when compiling for Darwin/PowerPC and 1 when
compiling for Darwin/x86, so this option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to generate code
that is not binary compatible with code generated without that
switch. Using this switch may require recompiling all other
modules in a program, including system libraries. Use this switch
to conform to a non-default data model.
-mfix-and-continue-ffix-and-continue-findirect-data
Generate code suitable for fast turn around development. Needed to
enable gdb to dynamically load ".o" files into already running
programs. -findirect-data and -ffix-and-continue are provided for
backwards compatibility.
-all_load
Loads all members of static archive libraries. See man ld(1) for
more information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong
architecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic linker
will bind all undefined references when the file is loaded or
launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more
information.
-bundle_loaderexecutable
This option specifies the executable that will be loading the build
output file being linked. See man ld(1) for more information.
-dynamiclib
When passed this option, GCC will produce a dynamic library instead
of an executable when linking, using the Darwin libtool command.
-force_cpusubtype_ALL
This causes GCC's output file to have the ALL subtype, instead of
one controlled by the -mcpu or -march option.
-allowable_clientclient_name-client_name-compatibility_version-current_version-dead_strip-dependency-file-dylib_file-dylinker_install_name-dynamic-exported_symbols_list-filelist-flat_namespace-force_flat_namespace-headerpad_max_install_names-image_base-init-install_name-keep_private_externs-multi_module-multiply_defined-multiply_defined_unused-noall_load-no_dead_strip_inits_and_terms-nofixprebinding-nomultidefs-noprebind-noseglinkedit-pagezero_size-prebind-prebind_all_twolevel_modules-private_bundle-read_only_relocs-sectalign-sectobjectsymbols-whyload-seg1addr-sectcreate-sectobjectsymbols-sectorder-segaddr-segs_read_only_addr-segs_read_write_addr-seg_addr_table-seg_addr_table_filename-seglinkedit-segprot-segs_read_only_addr-segs_read_write_addr-single_module-static-sub_library-sub_umbrella-twolevel_namespace-umbrella-undefined-unexported_symbols_list-weak_reference_mismatches-whatsloaded
These options are passed to the Darwin linker. The Darwin linker
man page describes them in detail.
DECAlphaOptions
These -m options are defined for the DEC Alpha implementations:
-mno-soft-float-msoft-float
Use (do not use) the hardware floating-point instructions for
floating-point operations. When -msoft-float is specified,
functions in libgcc.a will be used to perform floating-point
operations. Unless they are replaced by routines that emulate the
floating-point operations, or compiled in such a way as to call
such emulations routines, these routines will issue floating-point
operations. If you are compiling for an Alpha without floating-
point operations, you must ensure that the library is built so as
not to call them.
Note that Alpha implementations without floating-point operations
are required to have floating-point registers.
-mfp-reg-mno-fp-regs
Generate code that uses (does not use) the floating-point register
set. -mno-fp-regs implies -msoft-float. If the floating-point
register set is not used, floating-point operands are passed in
integer registers as if they were integers and floating-point
results are passed in $0 instead of $f0. This is a non-standard
calling sequence, so any function with a floating-point argument or
return value called by code compiled with -mno-fp-regs must also be
compiled with that option.
A typical use of this option is building a kernel that does not
use, and hence need not save and restore, any floating-point
registers.
-mieee
The Alpha architecture implements floating-point hardware optimized
for maximum performance. It is mostly compliant with the IEEE
floating-point standard. However, for full compliance, software
assistance is required. This option generates code fully IEEE-
compliant code except that the inexact-flag is not maintained (see
below). If this option is turned on, the preprocessor macro
"_IEEE_FP" is defined during compilation. The resulting code is
less efficient but is able to correctly support denormalized
numbers and exceptional IEEE values such as not-a-number and
plus/minus infinity. Other Alpha compilers call this option
-ieee_with_no_inexact.
-mieee-with-inexact
This is like -mieee except the generated code also maintains the
IEEE inexact-flag. Turning on this option causes the generated
code to implement fully-compliant IEEE math. In addition to
"_IEEE_FP", "_IEEE_FP_EXACT" is defined as a preprocessor macro.
On some Alpha implementations the resulting code may execute
significantly slower than the code generated by default. Since
there is very little code that depends on the inexact-flag, you
should normally not specify this option. Other Alpha compilers
call this option -ieee_with_inexact.
-mfp-trap-mode=trap-mode
This option controls what floating-point related traps are enabled.
Other Alpha compilers call this option -fptmtrap-mode. The trap
mode can be set to one of four values:
n This is the default (normal) setting. The only traps that are
enabled are the ones that cannot be disabled in software (e.g.,
division by zero trap).
u In addition to the traps enabled by n, underflow traps are
enabled as well.
su Like u, but the instructions are marked to be safe for software
completion (see Alpha architecture manual for details).
sui Like su, but inexact traps are enabled as well.
-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call this
option -fprmrounding-mode. The rounding-mode can be one of:
n Normal IEEE rounding mode. Floating-point numbers are rounded
towards the nearest machine number or towards the even machine
number in case of a tie.
m Round towards minus infinity.
c Chopped rounding mode. Floating-point numbers are rounded
towards zero.
d Dynamic rounding mode. A field in the floating-point control
register (fpcr, see Alpha architecture reference manual)
controls the rounding mode in effect. The C library
initializes this register for rounding towards plus infinity.
Thus, unless your program modifies the fpcr, d corresponds to
round towards plus infinity.
-mtrap-precision=trap-precision
In the Alpha architecture, floating-point traps are imprecise.
This means without software assistance it is impossible to recover
from a floating trap and program execution normally needs to be
terminated. GCC can generate code that can assist operating system
trap handlers in determining the exact location that caused a
floating-point trap. Depending on the requirements of an
application, different levels of precisions can be selected:
p Program precision. This option is the default and means a trap
handler can only identify which program caused a floating-point
exception.
f Function precision. The trap handler can determine the
function that caused a floating-point exception.
i Instruction precision. The trap handler can determine the
exact instruction that caused a floating-point exception.
Other Alpha compilers provide the equivalent options called
-scope_safe and -resumption_safe.
-mieee-conformant
This option marks the generated code as IEEE conformant. You must
not use this option unless you also specify -mtrap-precision=i and
either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is
to emit the line .eflag48 in the function prologue of the
generated assembly file. Under DEC Unix, this has the effect that
IEEE-conformant math library routines will be linked in.
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to see if it
can construct it from smaller constants in two or three
instructions. If it cannot, it will output the constant as a
literal and generate code to load it from the data segment at run
time.
Use this option to require GCC to construct all integer constants
using code, even if it takes more instructions (the maximum is
six).
You would typically use this option to build a shared library
dynamic loader. Itself a shared library, it must relocate itself
in memory before it can find the variables and constants in its own
data segment.
-malpha-as-mgas
Select whether to generate code to be assembled by the vendor-
supplied assembler (-malpha-as) or by the GNU assembler -mgas.
-mbwx-mno-bwx-mcix-mno-cix-mfix-mno-fix-mmax-mno-max
Indicate whether GCC should generate code to use the optional BWX,
CIX, FIX and MAX instruction sets. The default is to use the
instruction sets supported by the CPU type specified via -mcpu=
option or that of the CPU on which GCC was built if none was
specified.
-mfloat-vax-mfloat-ieee
Generate code that uses (does not use) VAX F and G floating-point
arithmetic instead of IEEE single and double precision.
-mexplicit-relocs-mno-explicit-relocs
Older Alpha assemblers provided no way to generate symbol
relocations except via assembler macros. Use of these macros does
not allow optimal instruction scheduling. GNU binutils as of
version 2.12 supports a new syntax that allows the compiler to
explicitly mark which relocations should apply to which
instructions. This option is mostly useful for debugging, as GCC
detects the capabilities of the assembler when it is built and sets
the default accordingly.
-msmall-data-mlarge-data
When -mexplicit-relocs is in effect, static data is accessed via
gp-relative relocations. When -msmall-data is used, objects 8
bytes long or smaller are placed in a smalldataarea (the ".sdata"
and ".sbss" sections) and are accessed via 16-bit relocations off
of the $gp register. This limits the size of the small data area
to 64KB, but allows the variables to be directly accessed via a
single instruction.
The default is -mlarge-data. With this option the data area is
limited to just below 2GB. Programs that require more than 2GB of
data must use "malloc" or "mmap" to allocate the data in the heap
instead of in the program's data segment.
When generating code for shared libraries, -fpic implies
-msmall-data and -fPIC implies -mlarge-data.
-msmall-text-mlarge-text
When -msmall-text is used, the compiler assumes that the code of
the entire program (or shared library) fits in 4MB, and is thus
reachable with a branch instruction. When -msmall-data is used,
the compiler can assume that all local symbols share the same $gp
value, and thus reduce the number of instructions required for a
function call from 4 to 1.
The default is -mlarge-text.
-mcpu=cpu_type
Set the instruction set and instruction scheduling parameters for
machine type cpu_type. You can specify either the EV style name or
the corresponding chip number. GCC supports scheduling parameters
for the EV4, EV5 and EV6 family of processors and will choose the
default values for the instruction set from the processor you
specify. If you do not specify a processor type, GCC will default
to the processor on which the compiler was built.
Supported values for cpu_type are
ev4ev4521064
Schedules as an EV4 and has no instruction set extensions.
ev521164
Schedules as an EV5 and has no instruction set extensions.
ev5621164a
Schedules as an EV5 and supports the BWX extension.
pca5621164pc21164PC
Schedules as an EV5 and supports the BWX and MAX extensions.
ev621264
Schedules as an EV6 and supports the BWX, FIX, and MAX
extensions.
ev6721264a
Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX
extensions.
Native toolchains also support the value native, which selects the
best architecture option for the host processor. -mcpu=native has
no effect if GCC does not recognize the processor.
-mtune=cpu_type
Set only the instruction scheduling parameters for machine type
cpu_type. The instruction set is not changed.
Native toolchains also support the value native, which selects the
best architecture option for the host processor. -mtune=native has
no effect if GCC does not recognize the processor.
-mmemory-latency=time
Sets the latency the scheduler should assume for typical memory
references as seen by the application. This number is highly
dependent on the memory access patterns used by the application and
the size of the external cache on the machine.
Valid options for time are
number
A decimal number representing clock cycles.
L1L2L3main
The compiler contains estimates of the number of clock cycles
for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches
(also called Dcache, Scache, and Bcache), as well as to main
memory. Note that L3 is only valid for EV5.
DECAlpha/VMSOptions
These -m options are defined for the DEC Alpha/VMS implementations:
-mvms-return-codes
Return VMS condition codes from main. The default is to return
POSIX style condition (e.g. error) codes.
-mdebug-main=prefix
Flag the first routine whose name starts with prefix as the main
routine for the debugger.
-mmalloc64
Default to 64-bit memory allocation routines.
FR30Options
These options are defined specifically for the FR30 port.
-msmall-model
Use the small address space model. This can produce smaller code,
but it does assume that all symbolic values and addresses will fit
into a 20-bit range.
-mno-lsim
Assume that runtime support has been provided and so there is no
need to include the simulator library (libsim.a) on the linker
command line.
FRVOptions-mgpr-32
Only use the first 32 general-purpose registers.
-mgpr-64
Use all 64 general-purpose registers.
-mfpr-32
Use only the first 32 floating-point registers.
-mfpr-64
Use all 64 floating-point registers.
-mhard-float
Use hardware instructions for floating-point operations.
-msoft-float
Use library routines for floating-point operations.
-malloc-cc
Dynamically allocate condition code registers.
-mfixed-cc
Do not try to dynamically allocate condition code registers, only
use "icc0" and "fcc0".
-mdword
Change ABI to use double word insns.
-mno-dword
Do not use double word instructions.
-mdouble
Use floating-point double instructions.
-mno-double
Do not use floating-point double instructions.
-mmedia
Use media instructions.
-mno-media
Do not use media instructions.
-mmuladd
Use multiply and add/subtract instructions.
-mno-muladd
Do not use multiply and add/subtract instructions.
-mfdpic
Select the FDPIC ABI, which uses function descriptors to represent
pointers to functions. Without any PIC/PIE-related options, it
implies -fPIE. With -fpic or -fpie, it assumes GOT entries and
small data are within a 12-bit range from the GOT base address;
with -fPIC or -fPIE, GOT offsets are computed with 32 bits. With a
bfin-elf target, this option implies -msim.
-minline-plt
Enable inlining of PLT entries in function calls to functions that
are not known to bind locally. It has no effect without -mfdpic.
It's enabled by default if optimizing for speed and compiling for
shared libraries (i.e., -fPIC or -fpic), or when an optimization
option such as -O3 or above is present in the command line.
-mTLS
Assume a large TLS segment when generating thread-local code.
-mtls
Do not assume a large TLS segment when generating thread-local
code.
-mgprel-ro
Enable the use of "GPREL" relocations in the FDPIC ABI for data
that is known to be in read-only sections. It's enabled by
default, except for -fpic or -fpie: even though it may help make
the global offset table smaller, it trades 1 instruction for 4.
With -fPIC or -fPIE, it trades 3 instructions for 4, one of which
may be shared by multiple symbols, and it avoids the need for a GOT
entry for the referenced symbol, so it's more likely to be a win.
If it is not, -mno-gprel-ro can be used to disable it.
-multilib-library-pic
Link with the (library, not FD) pic libraries. It's implied by
-mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic. You
should never have to use it explicitly.
-mlinked-fp
Follow the EABI requirement of always creating a frame pointer
whenever a stack frame is allocated. This option is enabled by
default and can be disabled with -mno-linked-fp.
-mlong-calls
Use indirect addressing to call functions outside the current
compilation unit. This allows the functions to be placed anywhere
within the 32-bit address space.
-malign-labels
Try to align labels to an 8-byte boundary by inserting nops into
the previous packet. This option only has an effect when VLIW
packing is enabled. It doesn't create new packets; it merely adds
nops to existing ones.
-mlibrary-pic
Generate position-independent EABI code.
-macc-4
Use only the first four media accumulator registers.
-macc-8
Use all eight media accumulator registers.
-mpack
Pack VLIW instructions.
-mno-pack
Do not pack VLIW instructions.
-mno-eflags
Do not mark ABI switches in e_flags.
-mcond-move
Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mno-cond-move
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mscc
Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mno-scc
Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mcond-exec
Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mno-cond-exec
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mvliw-branch
Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mno-vliw-branch
Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mmulti-cond-exec
Enable optimization of "&&" and "||" in conditional execution
(default).
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mno-multi-cond-exec
Disable optimization of "&&" and "||" in conditional execution.
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mnested-cond-exec
Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mno-nested-cond-exec
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-moptimize-membar
This switch removes redundant "membar" instructions from the
compiler generated code. It is enabled by default.
-mno-optimize-membar
This switch disables the automatic removal of redundant "membar"
instructions from the generated code.
-mtomcat-stats
Cause gas to print out tomcat statistics.
-mcpu=cpu
Select the processor type for which to generate code. Possible
values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300
and simple.
GNU/LinuxOptions
These -m options are defined for GNU/Linux targets:
-mglibc
Use the GNU C library. This is the default except on
*-*-linux-*uclibc* and *-*-linux-*android* targets.
-muclibc
Use uClibc C library. This is the default on *-*-linux-*uclibc*
targets.
-mbionic
Use Bionic C library. This is the default on *-*-linux-*android*
targets.
-mandroid
Compile code compatible with Android platform. This is the default
on *-*-linux-*android* targets.
When compiling, this option enables -mbionic, -fPIC,
-fno-exceptions and -fno-rtti by default. When linking, this
option makes the GCC driver pass Android-specific options to the
linker. Finally, this option causes the preprocessor macro
"__ANDROID__" to be defined.
-tno-android-cc
Disable compilation effects of -mandroid, i.e., do not enable
-mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
-tno-android-ld
Disable linking effects of -mandroid, i.e., pass standard Linux
linking options to the linker.
H8/300Options
These -m options are defined for the H8/300 implementations:
-mrelax
Shorten some address references at link time, when possible; uses
the linker option -relax.
-mh Generate code for the H8/300H.
-ms Generate code for the H8S.
-mn Generate code for the H8S and H8/300H in the normal mode. This
switch must be used either with -mh or -ms.
-ms2600
Generate code for the H8S/2600. This switch must be used with -ms.
-mint32
Make "int" data 32 bits by default.
-malign-300
On the H8/300H and H8S, use the same alignment rules as for the
H8/300. The default for the H8/300H and H8S is to align longs and
floats on 4-byte boundaries. -malign-300 causes them to be aligned
on 2-byte boundaries. This option has no effect on the H8/300.
HPPAOptions
These -m options are defined for the HPPA family of computers:
-march=architecture-type
Generate code for the specified architecture. The choices for
architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for
PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX
system to determine the proper architecture option for your
machine. Code compiled for lower numbered architectures will run
on higher numbered architectures, but not the other way around.
-mpa-risc-1-0-mpa-risc-1-1-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
-mbig-switch
Generate code suitable for big switch tables. Use this option only
if the assembler/linker complain about out of range branches within
a switch table.
-mjump-in-delay
Fill delay slots of function calls with unconditional jump
instructions by modifying the return pointer for the function call
to be the target of the conditional jump.
-mdisable-fpregs
Prevent floating-point registers from being used in any manner.
This is necessary for compiling kernels that perform lazy context
switching of floating-point registers. If you use this option and
attempt to perform floating-point operations, the compiler aborts.
-mdisable-indexing
Prevent the compiler from using indexing address modes. This
avoids some rather obscure problems when compiling MIG generated
code under MACH.
-mno-space-regs
Generate code that assumes the target has no space registers. This
allows GCC to generate faster indirect calls and use unscaled index
address modes.
Such code is suitable for level 0 PA systems and kernels.
-mfast-indirect-calls
Generate code that assumes calls never cross space boundaries.
This allows GCC to emit code that performs faster indirect calls.
This option will not work in the presence of shared libraries or
nested functions.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers.
A fixed register is one that the register allocator can not use.
This is useful when compiling kernel code. A register range is
specified as two registers separated by a dash. Multiple register
ranges can be specified separated by a comma.
-mlong-load-store
Generate 3-instruction load and store sequences as sometimes
required by the HP-UX 10 linker. This is equivalent to the +k
option to the HP compilers.
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF
systems.
-mgas
Enable the use of assembler directives only GAS understands.
-mschedule=cpu-type
Schedule code according to the constraints for the machine type
cpu-type. The choices for cpu-type are 7007100, 7100LC, 7200,
7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system
to determine the proper scheduling option for your machine. The
default scheduling is 8000.
-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note this makes
symbolic debugging impossible. It also triggers a bug in the HP-UX
8 and HP-UX 9 linkers in which they give bogus error messages when
linking some programs.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all HPPA
targets. Normally the facilities of the machine's usual C compiler
are used, but this cannot be done directly in cross-compilation.
You must make your own arrangements to provide suitable library
functions for cross-compilation.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this to
work.
-msio
Generate the predefine, "_SIO", for server IO. The default is
-mwsio. This generates the predefines, "__hp9000s700",
"__hp9000s700__" and "_WSIO", for workstation IO. These options
are available under HP-UX and HI-UX.
-mgnu-ld
Use GNU ld specific options. This passes -shared to ld when
building a shared library. It is the default when GCC is
configured, explicitly or implicitly, with the GNU linker. This
option does not have any affect on which ld is called, it only
changes what parameters are passed to that ld. The ld that is
called is determined by the --with-ld configure option, GCC's
program search path, and finally by the user's PATH. The linker
used by GCC can be printed using which`gcc-print-prog-name=ld`.
This option is only available on the 64-bit HP-UX GCC, i.e.
configured with hppa*64*-*-hpux*.
-mhp-ld
Use HP ld specific options. This passes -b to ld when building a
shared library and passes +AcceptTypeMismatch to ld on all links.
It is the default when GCC is configured, explicitly or implicitly,
with the HP linker. This option does not have any affect on which
ld is called, it only changes what parameters are passed to that
ld. The ld that is called is determined by the --with-ld configure
option, GCC's program search path, and finally by the user's PATH.
The linker used by GCC can be printed using which`gcc-print-prog-name=ld`. This option is only available on the 64-bit
HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
-mlong-calls
Generate code that uses long call sequences. This ensures that a
call is always able to reach linker generated stubs. The default
is to generate long calls only when the distance from the call site
to the beginning of the function or translation unit, as the case
may be, exceeds a predefined limit set by the branch type being
used. The limits for normal calls are 7,600,000 and 240,000 bytes,
respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are
always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using
the -ffunction-sections option, or when using the -mgas and
-mno-portable-runtime options together under HP-UX with the SOM
linker.
It is normally not desirable to use this option as it will degrade
performance. However, it may be useful in large applications,
particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the
assembler and linker, and the type of code being generated. The
impact on systems that support long absolute calls, and long pic
symbol-difference or pc-relative calls should be relatively small.
However, an indirect call is used on 32-bit ELF systems in pic code
and it is quite long.
-munix=unix-std
Generate compiler predefines and select a startfile for the
specified UNIX standard. The choices for unix-std are 93, 95 and
98. 93 is supported on all HP-UX versions. 95 is available on HP-
UX 10.10 and later. 98 is available on HP-UX 11.11 and later. The
default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 though to
11.00, and 98 for HP-UX 11.11 and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4.
-munix=95 provides additional predefines for "XOPEN_UNIX" and
"_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98
provides additional predefines for "_XOPEN_UNIX",
"_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and
"_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It is important to note that this option changes the interfaces for
various library routines. It also affects the operational behavior
of the C library. Thus, extreme care is needed in using this
option.
Library code that is intended to operate with more than one UNIX
standard must test, set and restore the variable
__xpg4_extended_mask as appropriate. Most GNU software doesn't
provide this capability.
-nolibdld
Suppress the generation of link options to search libdld.sl when
the -static option is specified on HP-UX 10 and later.
-static
The HP-UX implementation of setlocale in libc has a dependency on
libdld.sl. There isn't an archive version of libdld.sl. Thus,
when the -static option is specified, special link options are
needed to resolve this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to
link with libdld.sl when the -static option is specified. This
causes the resulting binary to be dynamic. On the 64-bit port, the
linkers generate dynamic binaries by default in any case. The
-nolibdld option can be used to prevent the GCC driver from adding
these link options.
-threads
Add support for multithreading with the dcethread library under
HP-UX. This option sets flags for both the preprocessor and
linker.
Intel386andAMDx86-64Options
These -m options are defined for the i386 and x86-64 family of
computers:
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code,
except for the ABI and the set of available instructions. The
choices for cpu-type are:
generic
Produce code optimized for the most common IA32/AMD64/EM64T
processors. If you know the CPU on which your code will run,
then you should use the corresponding -mtune option instead of
-mtune=generic. But, if you do not know exactly what CPU users
of your application will have, then you should use this option.
As new processors are deployed in the marketplace, the behavior
of this option will change. Therefore, if you upgrade to a
newer version of GCC, the code generated option will change to
reflect the processors that were most common when that version
of GCC was released.
There is no -march=generic option because -march indicates the
instruction set the compiler can use, and there is no generic
instruction set applicable to all processors. In contrast,
-mtune indicates the processor (or, in this case, collection of
processors) for which the code is optimized.
native
This selects the CPU to tune for at compilation time by
determining the processor type of the compiling machine. Using
-mtune=native will produce code optimized for the local machine
under the constraints of the selected instruction set. Using
-march=native will enable all instruction subsets supported by
the local machine (hence the result might not run on different
machines).
i386
Original Intel's i386 CPU.
i486
Intel's i486 CPU. (No scheduling is implemented for this
chip.)
i586,pentium
Intel Pentium CPU with no MMX support.
pentium-mmx
Intel PentiumMMX CPU based on Pentium core with MMX instruction
set support.
pentiumpro
Intel PentiumPro CPU.
i686
Same as "generic", but when used as "march" option, PentiumPro
instruction set will be used, so the code will run on all i686
family chips.
pentium2
Intel Pentium2 CPU based on PentiumPro core with MMX
instruction set support.
pentium3,pentium3m
Intel Pentium3 CPU based on PentiumPro core with MMX and SSE
instruction set support.
pentium-m
Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2
instruction set support. Used by Centrino notebooks.
pentium4,pentium4m
Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set
support.
prescott
Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and
SSE3 instruction set support.
nocona
Improved version of Intel Pentium4 CPU with 64-bit extensions,
MMX, SSE, SSE2 and SSE3 instruction set support.
core2
Intel Core2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3
and SSSE3 instruction set support.
corei7
Intel Core i7 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1 and SSE4.2 instruction set support.
corei7-avx
Intel Core i7 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, AVX, AES and PCLMUL instruction set
support.
core-avx-i
Intel Core CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, AVX, AES, PCLMUL, FSGSBASE, RDRND and
F16C instruction set support.
atom
Intel Atom CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and
SSSE3 instruction set support.
k6 AMD K6 CPU with MMX instruction set support.
k6-2,k6-3
Improved versions of AMD K6 CPU with MMX and 3DNow! instruction
set support.
athlon,athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE
prefetch instructions support.
athlon-4,athlon-xp,athlon-mp
Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and
full SSE instruction set support.
k8,opteron,athlon64,athlon-fx
AMD K8 core based CPUs with x86-64 instruction set support.
(This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and
64-bit instruction set extensions.)
k8-sse3,opteron-sse3,athlon64-sse3
Improved versions of k8, opteron and athlon64 with SSE3
instruction set support.
amdfam10,barcelona
AMD Family 10h core based CPUs with x86-64 instruction set
support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!,
enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
bdver1
AMD Family 15h core based CPUs with x86-64 instruction set
support. (This supersets FMA4, AVX, XOP, LWP, AES, PCL_MUL,
CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM
and 64-bit instruction set extensions.)
bdver2
AMD Family 15h core based CPUs with x86-64 instruction set
support. (This supersets BMI, TBM, F16C, FMA, AVX, XOP, LWP,
AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1,
SSE4.2, ABM and 64-bit instruction set extensions.)
btver1
AMD Family 14h core based CPUs with x86-64 instruction set
support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A,
CX16, ABM and 64-bit instruction set extensions.)
winchip-c6
IDT Winchip C6 CPU, dealt in same way as i486 with additional
MMX instruction set support.
winchip2
IDT Winchip2 CPU, dealt in same way as i486 with additional MMX
and 3DNow! instruction set support.
c3 Via C3 CPU with MMX and 3DNow! instruction set support. (No
scheduling is implemented for this chip.)
c3-2
Via C3-2 CPU with MMX and SSE instruction set support. (No
scheduling is implemented for this chip.)
geode
Embedded AMD CPU with MMX and 3DNow! instruction set support.
While picking a specific cpu-type will schedule things
appropriately for that particular chip, the compiler will not
generate any code that does not run on the default machine type
without the -march=cpu-type option being used. For example, if GCC
is configured for i686-pc-linux-gnu then -mtune=pentium4 will
generate code that is tuned for Pentium4 but will still run on i686
machines.
-march=cpu-type
Generate instructions for the machine type cpu-type. The choices
for cpu-type are the same as for -mtune. Moreover, specifying
-march=cpu-type implies -mtune=cpu-type.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-mfpmath=unit
Generate floating-point arithmetic for selected unit unit. The
choices for unit are:
387 Use the standard 387 floating-point coprocessor present on the
majority of chips and emulated otherwise. Code compiled with
this option runs almost everywhere. The temporary results are
computed in 80-bit precision instead of the precision specified
by the type, resulting in slightly different results compared
to most of other chips. See -ffloat-store for more detailed
description.
This is the default choice for i386 compiler.
sse Use scalar floating-point instructions present in the SSE
instruction set. This instruction set is supported by Pentium3
and newer chips, in the AMD line by Athlon-4, Athlon-xp and
Athlon-mp chips. The earlier version of SSE instruction set
supports only single-precision arithmetic, thus the double and
extended-precision arithmetic are still done using 387. A
later version, present only in Pentium4 and the future AMD
x86-64 chips, supports double-precision arithmetic too.
For the i386 compiler, you need to use -march=cpu-type, -msse
or -msse2 switches to enable SSE extensions and make this
option effective. For the x86-64 compiler, these extensions
are enabled by default.
The resulting code should be considerably faster in the
majority of cases and avoid the numerical instability problems
of 387 code, but may break some existing code that expects
temporaries to be 80 bits.
This is the default choice for the x86-64 compiler.
sse,387sse+387both
Attempt to utilize both instruction sets at once. This
effectively double the amount of available registers and on
chips with separate execution units for 387 and SSE the
execution resources too. Use this option with care, as it is
still experimental, because the GCC register allocator does not
model separate functional units well resulting in instable
performance.
-masm=dialect
Output asm instructions using selected dialect. Supported choices
are intel or att (the default one). Darwin does not support intel.
-mieee-fp-mno-ieee-fp
Control whether or not the compiler uses IEEE floating-point
comparisons. These handle correctly the case where the result of a
comparison is unordered.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GCC. Normally the
facilities of the machine's usual C compiler are used, but this
can't be done directly in cross-compilation. You must make your
own arrangements to provide suitable library functions for cross-
compilation.
On machines where a function returns floating-point results in the
80387 register stack, some floating-point opcodes may be emitted
even if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types
"float" and "double" in an FPU register, even if there is no FPU.
The idea is that the operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned in
ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt"
instructions for the 387. Specify this option to avoid generating
those instructions. This option is the default on FreeBSD, OpenBSD
and NetBSD. This option is overridden when -march indicates that
the target CPU will always have an FPU and so the instruction will
not need emulation. As of revision 2.6.1, these instructions are
not generated unless you also use the -funsafe-math-optimizations
switch.
-malign-double-mno-align-double
Control whether GCC aligns "double", "long double", and "long long"
variables on a two-word boundary or a one-word boundary. Aligning
"double" variables on a two-word boundary produces code that runs
somewhat faster on a Pentium at the expense of more memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch, structures
containing the above types will be aligned differently than the
published application binary interface specifications for the 386
and will not be binary compatible with structures in code compiled
without that switch.
-m96bit-long-double-m128bit-long-double
These switches control the size of "long double" type. The i386
application binary interface specifies the size to be 96 bits, so
-m96bit-long-double is the default in 32-bit mode.
Modern architectures (Pentium and newer) prefer "long double" to be
aligned to an 8- or 16-byte boundary. In arrays or structures
conforming to the ABI, this is not possible. So specifying
-m128bit-long-double aligns "long double" to a 16-byte boundary by
padding the "long double" with an additional 32-bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice
as its ABI specifies that "long double" is to be aligned on 16-byte
boundary.
Notice that neither of these options enable any extra precision
over the x87 standard of 80 bits for a "long double".
Warning: if you override the default value for your target ABI, the
structures and arrays containing "long double" variables will
change their size as well as function calling convention for
function taking "long double" will be modified. Hence they will
not be binary compatible with arrays or structures in code compiled
without that switch.
-mlarge-data-threshold=number
When -mcmodel=medium is specified, the data greater than threshold
are placed in large data section. This value must be the same
across all object linked into the binary and defaults to 65535.
-mrtd
Use a different function-calling convention, in which functions
that take a fixed number of arguments return with the "ret" num
instruction, which pops their arguments while returning. This
saves one instruction in the caller since there is no need to pop
the arguments there.
You can specify that an individual function is called with this
calling sequence with the function attribute stdcall. You can also
override the -mrtd option by using the function attribute cdecl.
Warning: this calling convention is incompatible with the one
normally used on Unix, so you cannot use it if you need to call
libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including "printf"); otherwise
incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
-mregparm=num
Control how many registers are used to pass integer arguments. By
default, no registers are used to pass arguments, and at most 3
registers can be used. You can control this behavior for a
specific function by using the function attribute regparm.
Warning: if you use this switch, and num is nonzero, then you must
build all modules with the same value, including any libraries.
This includes the system libraries and startup modules.
-msseregparm
Use SSE register passing conventions for float and double arguments
and return values. You can control this behavior for a specific
function by using the function attribute sseregparm.
Warning: if you use this switch then you must build all modules
with the same value, including any libraries. This includes the
system libraries and startup modules.
-mvect8-ret-in-mem
Return 8-byte vectors in memory instead of MMX registers. This is
the default on Solaris@tie{}8 and 9 and VxWorks to match the ABI of
the Sun Studio compilers until version 12. Later compiler versions
(starting with Studio 12 Update@tie{}1) follow the ABI used by
other x86 targets, which is the default on Solaris@tie{}10 and
later. Only use this option if you need to remain compatible with
existing code produced by those previous compiler versions or older
versions of GCC.
-mpc32-mpc64-mpc80
Set 80387 floating-point precision to 32, 64 or 80 bits. When
-mpc32 is specified, the significands of results of floating-point
operations are rounded to 24 bits (single precision); -mpc64 rounds
the significands of results of floating-point operations to 53 bits
(double precision) and -mpc80 rounds the significands of results of
floating-point operations to 64 bits (extended double precision),
which is the default. When this option is used, floating-point
operations in higher precisions are not available to the programmer
without setting the FPU control word explicitly.
Setting the rounding of floating-point operations to less than the
default 80 bits can speed some programs by 2% or more. Note that
some mathematical libraries assume that extended-precision (80-bit)
floating-point operations are enabled by default; routines in such
libraries could suffer significant loss of accuracy, typically
through so-called "catastrophic cancellation", when this option is
used to set the precision to less than extended precision.
-mstackrealign
Realign the stack at entry. On the Intel x86, the -mstackrealign
option will generate an alternate prologue and epilogue that
realigns the run-time stack if necessary. This supports mixing
legacy codes that keep a 4-byte aligned stack with modern codes
that keep a 16-byte stack for SSE compatibility. See also the
attribute "force_align_arg_pointer", applicable to individual
functions.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num
byte boundary. If -mpreferred-stack-boundary is not specified, the
default is 4 (16 bytes or 128 bits).
-mincoming-stack-boundary=num
Assume the incoming stack is aligned to a 2 raised to num byte
boundary. If -mincoming-stack-boundary is not specified, the one
specified by -mpreferred-stack-boundary will be used.
On Pentium and PentiumPro, "double" and "long double" values should
be aligned to an 8-byte boundary (see -malign-double) or suffer
significant run time performance penalties. On Pentium III, the
Streaming SIMD Extension (SSE) data type "__m128" may not work
properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack, the stack
boundary must be as aligned as that required by any value stored on
the stack. Further, every function must be generated such that it
keeps the stack aligned. Thus calling a function compiled with a
higher preferred stack boundary from a function compiled with a
lower preferred stack boundary will most likely misalign the stack.
It is recommended that libraries that use callbacks always use the
default setting.
This extra alignment does consume extra stack space, and generally
increases code size. Code that is sensitive to stack space usage,
such as embedded systems and operating system kernels, may want to
reduce the preferred alignment to -mpreferred-stack-boundary=2.
-mmmx-mno-mmx-msse-mno-sse-msse2-mno-sse2-msse3-mno-sse3-mssse3-mno-ssse3-msse4.1-mno-sse4.1-msse4.2-mno-sse4.2-msse4-mno-sse4-mavx-mno-avx-mavx2-mno-avx2-maes-mno-aes-mpclmul-mno-pclmul-mfsgsbase-mno-fsgsbase-mrdrnd-mno-rdrnd-mf16c-mno-f16c-mfma-mno-fma-msse4a-mno-sse4a-mfma4-mno-fma4-mxop-mno-xop-mlwp-mno-lwp-m3dnow-mno-3dnow-mpopcnt-mno-popcnt-mabm-mno-abm-mbmi-mbmi2-mno-bmi-mno-bmi2-mlzcnt-mno-lzcnt-mtbm-mno-tbm
These switches enable or disable the use of instructions in the
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, AVX, AVX2, AES, PCLMUL,
FSGSBASE, RDRND, F16C, FMA, SSE4A, FMA4, XOP, LWP, ABM, BMI, BMI2,
LZCNT or 3DNow!
extended instruction sets. These extensions are also available as
built-in functions: see X86Built-inFunctions, for details of the
functions enabled and disabled by these switches.
To have SSE/SSE2 instructions generated automatically from
floating-point code (as opposed to 387 instructions), see
-mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead, it
generates new AVX instructions or AVX equivalence for all SSEx
instructions when needed.
These options will enable GCC to use these extended instructions in
generated code, even without -mfpmath=sse. Applications that
perform run-time CPU detection must compile separate files for each
supported architecture, using the appropriate flags. In
particular, the file containing the CPU detection code should be
compiled without these options.
-mcld
This option instructs GCC to emit a "cld" instruction in the
prologue of functions that use string instructions. String
instructions depend on the DF flag to select between autoincrement
or autodecrement mode. While the ABI specifies the DF flag to be
cleared on function entry, some operating systems violate this
specification by not clearing the DF flag in their exception
dispatchers. The exception handler can be invoked with the DF flag
set, which leads to wrong direction mode when string instructions
are used. This option can be enabled by default on 32-bit x86
targets by configuring GCC with the --enable-cld configure option.
Generation of "cld" instructions can be suppressed with the
-mno-cld compiler option in this case.
-mvzeroupper
This option instructs GCC to emit a "vzeroupper" instruction before
a transfer of control flow out of the function to minimize AVX to
SSE transition penalty as well as remove unnecessary zeroupper
intrinsics.
-mprefer-avx128
This option instructs GCC to use 128-bit AVX instructions instead
of 256-bit AVX instructions in the auto-vectorizer.
-mcx16
This option will enable GCC to use CMPXCHG16B instruction in
generated code. CMPXCHG16B allows for atomic operations on 128-bit
double quadword (or oword) data types. This is useful for high
resolution counters that could be updated by multiple processors
(or cores). This instruction is generated as part of atomic built-
in functions: see __syncBuiltins or __atomicBuiltins for details.
-msahf
This option will enable GCC to use SAHF instruction in generated
64-bit code. Early Intel CPUs with Intel 64 lacked LAHF and SAHF
instructions supported by AMD64 until introduction of Pentium 4 G1
step in December 2005. LAHF and SAHF are load and store
instructions, respectively, for certain status flags. In 64-bit
mode, SAHF instruction is used to optimize "fmod", "drem" or
"remainder" built-in functions: see OtherBuiltins for details.
-mmovbe
This option will enable GCC to use movbe instruction to implement
"__builtin_bswap32" and "__builtin_bswap64".
-mcrc32
This option will enable built-in functions,
"__builtin_ia32_crc32qi", "__builtin_ia32_crc32hi".
"__builtin_ia32_crc32si" and "__builtin_ia32_crc32di" to generate
the crc32 machine instruction.
-mrecip
This option will enable GCC to use RCPSS and RSQRTSS instructions
(and their vectorized variants RCPPS and RSQRTPS) with an
additional Newton-Raphson step to increase precision instead of
DIVSS and SQRTSS (and their vectorized variants) for single-
precision floating-point arguments. These instructions are
generated only when -funsafe-math-optimizations is enabled together
with -finite-math-only and -fno-trapping-math. Note that while the
throughput of the sequence is higher than the throughput of the
non-reciprocal instruction, the precision of the sequence can be
decreased by up to 2 ulp (i.e. the inverse of 1.0 equals
0.99999994).
Note that GCC implements "1.0f/sqrtf(x)" in terms of RSQRTSS (or
RSQRTPS) already with -ffast-math (or the above option
combination), and doesn't need -mrecip.
Also note that GCC emits the above sequence with additional Newton-
Raphson step for vectorized single-float division and vectorized
"sqrtf(x)" already with -ffast-math (or the above option
combination), and doesn't need -mrecip.
-mrecip=opt
This option allows to control which reciprocal estimate
instructions may be used. opt is a comma separated list of
options, which may be preceded by a "!" to invert the option:
"all": enable all estimate instructions, "default": enable the
default instructions, equivalent to -mrecip, "none": disable all
estimate instructions, equivalent to -mno-recip, "div": enable the
approximation for scalar division, "vec-div": enable the
approximation for vectorized division, "sqrt": enable the
approximation for scalar square root, "vec-sqrt": enable the
approximation for vectorized square root.
So for example, -mrecip=all,!sqrt would enable all of the
reciprocal approximations, except for square root.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an
external library. Supported types are "svml" for the Intel short
vector math library and "acml" for the AMD math core library style
of interfacing. GCC will currently emit calls to "vmldExp2",
"vmldLn2", "vmldLog102", "vmldLog102", "vmldPow2", "vmldTanh2",
"vmldTan2", "vmldAtan2", "vmldAtanh2", "vmldCbrt2", "vmldSinh2",
"vmldSin2", "vmldAsinh2", "vmldAsin2", "vmldCosh2", "vmldCos2",
"vmldAcosh2", "vmldAcos2", "vmlsExp4", "vmlsLn4", "vmlsLog104",
"vmlsLog104", "vmlsPow4", "vmlsTanh4", "vmlsTan4", "vmlsAtan4",
"vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4", "vmlsSin4", "vmlsAsinh4",
"vmlsAsin4", "vmlsCosh4", "vmlsCos4", "vmlsAcosh4" and "vmlsAcos4"
for corresponding function type when -mveclibabi=svml is used and
"__vrd2_sin", "__vrd2_cos", "__vrd2_exp", "__vrd2_log",
"__vrd2_log2", "__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf",
"__vrs4_expf", "__vrs4_logf", "__vrs4_log2f", "__vrs4_log10f" and
"__vrs4_powf" for corresponding function type when -mveclibabi=acml
is used. Both -ftree-vectorize and -funsafe-math-optimizations have
to be enabled. A SVML or ACML ABI compatible library will have to
be specified at link time.
-mabi=name
Generate code for the specified calling convention. Permissible
values are: sysv for the ABI used on GNU/Linux and other systems
and ms for the Microsoft ABI. The default is to use the Microsoft
ABI when targeting Windows. On all other systems, the default is
the SYSV ABI. You can control this behavior for a specific
function by using the function attribute ms_abi/sysv_abi.
-mtls-dialect=type
Generate code to access thread-local storage using the gnu or gnu2
conventions. gnu is the conservative default; gnu2 is more
efficient, but it may add compile- and run-time requirements that
cannot be satisfied on all systems.
-mpush-args-mno-push-args
Use PUSH operations to store outgoing parameters. This method is
shorter and usually equally fast as method using SUB/MOV operations
and is enabled by default. In some cases disabling it may improve
performance because of improved scheduling and reduced
dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing
arguments will be computed in the function prologue. This is
faster on most modern CPUs because of reduced dependencies,
improved scheduling and reduced stack usage when preferred stack
boundary is not equal to 2. The drawback is a notable increase in
code size. This switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on Mingw32. Code that
relies on thread-safe exception handling must compile and link all
code with the -mthreads option. When compiling, -mthreads defines
-D_MT; when linking, it links in a special thread helper library
-lmingwthrd which cleans up per thread exception handling data.
-mno-align-stringops
Do not align destination of inlined string operations. This switch
reduces code size and improves performance in case the destination
is already aligned, but GCC doesn't know about it.
-minline-all-stringops
By default GCC inlines string operations only when the destination
is known to be aligned to least a 4-byte boundary. This enables
more inlining, increase code size, but may improve performance of
code that depends on fast memcpy, strlen and memset for short
lengths.
-minline-stringops-dynamically
For string operations of unknown size, use run-time checks with
inline code for small blocks and a library call for large blocks.
-mstringop-strategy=alg
Overwrite internal decision heuristic about particular algorithm to
inline string operation with. The allowed values are "rep_byte",
"rep_4byte", "rep_8byte" for expanding using i386 "rep" prefix of
specified size, "byte_loop", "loop", "unrolled_loop" for expanding
inline loop, "libcall" for always expanding library call.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions.
This avoids the instructions to save, set up and restore frame
pointers and makes an extra register available in leaf functions.
The option -fomit-frame-pointer removes the frame pointer for all
functions, which might make debugging harder.
-mtls-direct-seg-refs-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets from
the TLS segment register (%gs for 32-bit, %fs for 64-bit), or
whether the thread base pointer must be added. Whether or not this
is legal depends on the operating system, and whether it maps the
segment to cover the entire TLS area.
For systems that use GNU libc, the default is on.
-msse2avx-mno-sse2avx
Specify that the assembler should encode SSE instructions with VEX
prefix. The option -mavx turns this on by default.
-mfentry-mno-fentry
If profiling is active -pg put the profiling counter call before
prologue. Note: On x86 architectures the attribute
"ms_hook_prologue" isn't possible at the moment for -mfentry and
-pg.
-m8bit-idiv-mno-8bit-idiv
On some processors, like Intel Atom, 8-bit unsigned integer divide
is much faster than 32-bit/64-bit integer divide. This option
generates a run-time check. If both dividend and divisor are
within range of 0 to 255, 8-bit unsigned integer divide is used
instead of 32-bit/64-bit integer divide.
-mavx256-split-unaligned-load-mavx256-split-unaligned-store
Split 32-byte AVX unaligned load and store.
These -m switches are supported in addition to the above on AMD x86-64
processors in 64-bit environments.
-m32-m64-mx32
Generate code for a 32-bit or 64-bit environment. The -m32 option
sets int, long and pointer to 32 bits and generates code that runs
on any i386 system. The -m64 option sets int to 32 bits and long
and pointer to 64 bits and generates code for AMD's x86-64
architecture. The -mx32 option sets int, long and pointer to 32
bits and generates code for AMD's x86-64 architecture. For darwin
only the -m64 option turns off the -fno-pic and -mdynamic-no-pic
options.
-mno-red-zone
Do not use a so called red zone for x86-64 code. The red zone is
mandated by the x86-64 ABI, it is a 128-byte area beyond the
location of the stack pointer that will not be modified by signal
or interrupt handlers and therefore can be used for temporary data
without adjusting the stack pointer. The flag -mno-red-zone
disables this red zone.
-mcmodel=small
Generate code for the small code model: the program and its symbols
must be linked in the lower 2 GB of the address space. Pointers
are 64 bits. Programs can be statically or dynamically linked.
This is the default code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the
negative 2 GB of the address space. This model has to be used for
Linux kernel code.
-mcmodel=medium
Generate code for the medium model: The program is linked in the
lower 2 GB of the address space. Small symbols are also placed
there. Symbols with sizes larger than -mlarge-data-threshold are
put into large data or bss sections and can be located above 2GB.
Programs can be statically or dynamically linked.
-mcmodel=large
Generate code for the large model: This model makes no assumptions
about addresses and sizes of sections.
i386andx86-64WindowsOptions
These additional options are available for Windows targets:
-mconsole
This option is available for Cygwin and MinGW targets. It
specifies that a console application is to be generated, by
instructing the linker to set the PE header subsystem type required
for console applications. This is the default behavior for Cygwin
and MinGW targets.
-mdll
This option is available for Cygwin and MinGW targets. It
specifies that a DLL - a dynamic link library - is to be generated,
enabling the selection of the required runtime startup object and
entry point.
-mnop-fun-dllimport
This option is available for Cygwin and MinGW targets. It
specifies that the dllimport attribute should be ignored.
-mthread
This option is available for MinGW targets. It specifies that
MinGW-specific thread support is to be used.
-municode
This option is available for mingw-w64 targets. It specifies that
the UNICODE macro is getting pre-defined and that the unicode
capable runtime startup code is chosen.
-mwin32
This option is available for Cygwin and MinGW targets. It
specifies that the typical Windows pre-defined macros are to be set
in the pre-processor, but does not influence the choice of runtime
library/startup code.
-mwindows
This option is available for Cygwin and MinGW targets. It
specifies that a GUI application is to be generated by instructing
the linker to set the PE header subsystem type appropriately.
-fno-set-stack-executable
This option is available for MinGW targets. It specifies that the
executable flag for stack used by nested functions isn't set. This
is necessary for binaries running in kernel mode of Windows, as
there the user32 API, which is used to set executable privileges,
isn't available.
-mpe-aligned-commons
This option is available for Cygwin and MinGW targets. It
specifies that the GNU extension to the PE file format that permits
the correct alignment of COMMON variables should be used when
generating code. It will be enabled by default if GCC detects that
the target assembler found during configuration supports the
feature.
See also under i386andx86-64Options for standard options.
IA-64Options
These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian
Generate code for a big-endian target. This is the default for HP-
UX.
-mlittle-endian
Generate code for a little-endian target. This is the default for
AIX5 and GNU/Linux.
-mgnu-as-mno-gnu-as
Generate (or don't) code for the GNU assembler. This is the
default.
-mgnu-ld-mno-gnu-ld
Generate (or don't) code for the GNU linker. This is the default.
-mno-pic
Generate code that does not use a global pointer register. The
result is not position independent code, and violates the IA-64
ABI.
-mvolatile-asm-stop-mno-volatile-asm-stop
Generate (or don't) a stop bit immediately before and after
volatile asm statements.
-mregister-names-mno-register-names
Generate (or don't) in, loc, and out register names for the stacked
registers. This may make assembler output more readable.
-mno-sdata-msdata
Disable (or enable) optimizations that use the small data section.
This may be useful for working around optimizer bugs.
-mconstant-gp
Generate code that uses a single constant global pointer value.
This is useful when compiling kernel code.
-mauto-pic
Generate code that is self-relocatable. This implies
-mconstant-gp. This is useful when compiling firmware code.
-minline-float-divide-min-latency
Generate code for inline divides of floating-point values using the
minimum latency algorithm.
-minline-float-divide-max-throughput
Generate code for inline divides of floating-point values using the
maximum throughput algorithm.
-mno-inline-float-divide
Do not generate inline code for divides of floating-point values.
-minline-int-divide-min-latency
Generate code for inline divides of integer values using the
minimum latency algorithm.
-minline-int-divide-max-throughput
Generate code for inline divides of integer values using the
maximum throughput algorithm.
-mno-inline-int-divide
Do not generate inline code for divides of integer values.
-minline-sqrt-min-latency
Generate code for inline square roots using the minimum latency
algorithm.
-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum throughput
algorithm.
-mno-inline-sqrt
Do not generate inline code for sqrt.
-mfused-madd-mno-fused-madd
Do (don't) generate code that uses the fused multiply/add or
multiply/subtract instructions. The default is to use these
instructions.
-mno-dwarf2-asm-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF2 line number
debugging info. This may be useful when not using the GNU
assembler.
-mearly-stop-bits-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately preceding the
instruction that triggered the stop bit. This can improve
instruction scheduling, but does not always do so.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers.
A fixed register is one that the register allocator can not use.
This is useful when compiling kernel code. A register range is
specified as two registers separated by a dash. Multiple register
ranges can be specified separated by a comma.
-mtls-size=tls-size
Specify bit size of immediate TLS offsets. Valid values are 14,
22, and 64.
-mtune=cpu-type
Tune the instruction scheduling for a particular CPU, Valid values
are itanium, itanium1, merced, itanium2, and mckinley.
-milp32-mlp64
Generate code for a 32-bit or 64-bit environment. The 32-bit
environment sets int, long and pointer to 32 bits. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits.
These are HP-UX specific flags.
-mno-sched-br-data-spec-msched-br-data-spec
(Dis/En)able data speculative scheduling before reload. This will
result in generation of the ld.a instructions and the corresponding
check instructions (ld.c / chk.a). The default is 'disable'.
-msched-ar-data-spec-mno-sched-ar-data-spec
(En/Dis)able data speculative scheduling after reload. This will
result in generation of the ld.a instructions and the corresponding
check instructions (ld.c / chk.a). The default is 'enable'.
-mno-sched-control-spec-msched-control-spec
(Dis/En)able control speculative scheduling. This feature is
available only during region scheduling (i.e. before reload). This
will result in generation of the ld.s instructions and the
corresponding check instructions chk.s . The default is 'disable'.
-msched-br-in-data-spec-mno-sched-br-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are
dependent on the data speculative loads before reload. This is
effective only with -msched-br-data-spec enabled. The default is
'enable'.
-msched-ar-in-data-spec-mno-sched-ar-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are
dependent on the data speculative loads after reload. This is
effective only with -msched-ar-data-spec enabled. The default is
'enable'.
-msched-in-control-spec-mno-sched-in-control-spec
(En/Dis)able speculative scheduling of the instructions that are
dependent on the control speculative loads. This is effective only
with -msched-control-spec enabled. The default is 'enable'.
-mno-sched-prefer-non-data-spec-insns-msched-prefer-non-data-spec-insns
If enabled, data speculative instructions will be chosen for
schedule only if there are no other choices at the moment. This
will make the use of the data speculation much more conservative.
The default is 'disable'.
-mno-sched-prefer-non-control-spec-insns-msched-prefer-non-control-spec-insns
If enabled, control speculative instructions will be chosen for
schedule only if there are no other choices at the moment. This
will make the use of the control speculation much more
conservative. The default is 'disable'.
-mno-sched-count-spec-in-critical-path-msched-count-spec-in-critical-path
If enabled, speculative dependencies will be considered during
computation of the instructions priorities. This will make the use
of the speculation a bit more conservative. The default is
'disable'.
-msched-spec-ldc
Use a simple data speculation check. This option is on by default.
-msched-control-spec-ldc
Use a simple check for control speculation. This option is on by
default.
-msched-stop-bits-after-every-cycle
Place a stop bit after every cycle when scheduling. This option is
on by default.
-msched-fp-mem-deps-zero-cost
Assume that floating-point stores and loads are not likely to cause
a conflict when placed into the same instruction group. This
option is disabled by default.
-msel-sched-dont-check-control-spec
Generate checks for control speculation in selective scheduling.
This flag is disabled by default.
-msched-max-memory-insns=max-insns
Limit on the number of memory insns per instruction group, giving
lower priority to subsequent memory insns attempting to schedule in
the same instruction group. Frequently useful to prevent cache bank
conflicts. The default value is 1.
-msched-max-memory-insns-hard-limit
Disallow more than `msched-max-memory-insns' in instruction group.
Otherwise, limit is `soft' meaning that we would prefer non-memory
operations when limit is reached but may still schedule memory
operations.
IA-64/VMSOptions
These -m options are defined for the IA-64/VMS implementations:
-mvms-return-codes
Return VMS condition codes from main. The default is to return
POSIX style condition (e.g. error) codes.
-mdebug-main=prefix
Flag the first routine whose name starts with prefix as the main
routine for the debugger.
-mmalloc64
Default to 64-bit memory allocation routines.
LM32Options
These -m options are defined for the Lattice Mico32 architecture:
-mbarrel-shift-enabled
Enable barrel-shift instructions.
-mdivide-enabled
Enable divide and modulus instructions.
-mmultiply-enabled
Enable multiply instructions.
-msign-extend-enabled
Enable sign extend instructions.
-muser-enabled
Enable user-defined instructions.
M32COptions-mcpu=name
Select the CPU for which code is generated. name may be one of r8c
for the R8C/Tiny series, m16c for the M16C (up to /60) series,
m32cm for the M16C/80 series, or m32c for the M32C/80 series.
-msim
Specifies that the program will be run on the simulator. This
causes an alternate runtime library to be linked in which supports,
for example, file I/O. You must not use this option when
generating programs that will run on real hardware; you must
provide your own runtime library for whatever I/O functions are
needed.
-memregs=number
Specifies the number of memory-based pseudo-registers GCC will use
during code generation. These pseudo-registers will be used like
real registers, so there is a tradeoff between GCC's ability to fit
the code into available registers, and the performance penalty of
using memory instead of registers. Note that all modules in a
program must be compiled with the same value for this option.
Because of that, you must not use this option with the default
runtime libraries gcc builds.
M32R/DOptions
These -m options are defined for Renesas M32R/D architectures:
-m32r2
Generate code for the M32R/2.
-m32rx
Generate code for the M32R/X.
-m32r
Generate code for the M32R. This is the default.
-mmodel=small
Assume all objects live in the lower 16MB of memory (so that their
addresses can be loaded with the "ld24" instruction), and assume
all subroutines are reachable with the "bl" instruction. This is
the default.
The addressability of a particular object can be set with the
"model" attribute.
-mmodel=medium
Assume objects may be anywhere in the 32-bit address space (the
compiler will generate "seth/add3" instructions to load their
addresses), and assume all subroutines are reachable with the "bl"
instruction.
-mmodel=large
Assume objects may be anywhere in the 32-bit address space (the
compiler will generate "seth/add3" instructions to load their
addresses), and assume subroutines may not be reachable with the
"bl" instruction (the compiler will generate the much slower
"seth/add3/jl" instruction sequence).
-msdata=none
Disable use of the small data area. Variables will be put into one
of .data, bss, or .rodata (unless the "section" attribute has been
specified). This is the default.
The small data area consists of sections .sdata and .sbss. Objects
may be explicitly put in the small data area with the "section"
attribute using one of these sections.
-msdata=sdata
Put small global and static data in the small data area, but do not
generate special code to reference them.
-msdata=use
Put small global and static data in the small data area, and
generate special instructions to reference them.
-Gnum
Put global and static objects less than or equal to num bytes into
the small data or bss sections instead of the normal data or bss
sections. The default value of num is 8. The -msdata option must
be set to one of sdata or use for this option to have any effect.
All modules should be compiled with the same -Gnum value.
Compiling with different values of num may or may not work; if it
doesn't the linker will give an error message---incorrect code will
not be generated.
-mdebug
Makes the M32R specific code in the compiler display some
statistics that might help in debugging programs.
-malign-loops
Align all loops to a 32-byte boundary.
-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is the default.
-missue-rate=number
Issue number instructions per cycle. number can only be 1 or 2.
-mbranch-cost=numbernumber can only be 1 or 2. If it is 1 then branches will be
preferred over conditional code, if it is 2, then the opposite will
apply.
-mflush-trap=number
Specifies the trap number to use to flush the cache. The default
is 12. Valid numbers are between 0 and 15 inclusive.
-mno-flush-trap
Specifies that the cache cannot be flushed by using a trap.
-mflush-func=name
Specifies the name of the operating system function to call to
flush the cache. The default is _flush_cache, but a function call
will only be used if a trap is not available.
-mno-flush-func
Indicates that there is no OS function for flushing the cache.
M680x0Options
These are the -m options defined for M680x0 and ColdFire processors.
The default settings depend on which architecture was selected when the
compiler was configured; the defaults for the most common choices are
given below.
-march=arch
Generate code for a specific M680x0 or ColdFire instruction set
architecture. Permissible values of arch for M680x0 architectures
are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire
architectures are selected according to Freescale's ISA
classification and the permissible values are: isaa, isaaplus, isab
and isac.
gcc defines a macro __mcfarch__ whenever it is generating code for
a ColdFire target. The arch in this macro is one of the -march
arguments given above.
When used together, -march and -mtune select code that runs on a
family of similar processors but that is optimized for a particular
microarchitecture.
-mcpu=cpu
Generate code for a specific M680x0 or ColdFire processor. The
M680x0 cpus are: 68000, 68010, 68020, 68030, 68040, 68060, 68302,
68332 and cpu32. The ColdFire cpus are given by the table below,
which also classifies the CPUs into families:
Family : -mcpu arguments
51 : 5151ac51cn51em51qe5206 : 5202520452065206e : 5206e5208 : 520752085211a : 5210a5211a5213 : 5211521252135216 : 5214521652235 : 5223052231522325223352234522355225 : 5224522552259 : 5225252254522555225652258522595235 : 5232523352345235523x5249 : 52495250 : 52505271 : 527052715272 : 52725275 : 527452755282 : 528052815282528x53017 : 530115301253013530145301553016530175307 : 53075329 : 532753285329532x5373 : 53725373537x5407 : 54075475 : 547054715472547354745475547x548054815482548354845485-mcpu=cpu overrides -march=arch if arch is compatible with cpu.
Other combinations of -mcpu and -march are rejected.
gcc defines the macro __mcf_cpu_cpu when ColdFire target cpu is
selected. It also defines __mcf_family_family, where the value of
family is given by the table above.
-mtune=tune
Tune the code for a particular microarchitecture, within the
constraints set by -march and -mcpu. The M680x0 microarchitectures
are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The
ColdFire microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e.
You can also use -mtune=68020-40 for code that needs to run
relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60
is similar but includes 68060 targets as well. These two options
select the same tuning decisions as -m68020-40 and -m68020-60
respectively.
gcc defines the macros __mcarch and __mcarch__ when tuning for
680x0 architecture arch. It also defines mcarch unless either
-ansi or a non-GNU -std option is used. If gcc is tuning for a
range of architectures, as selected by -mtune=68020-40 or
-mtune=68020-60, it defines the macros for every architecture in
the range.
gcc also defines the macro __muarch__ when tuning for ColdFire
microarchitecture uarch, where uarch is one of the arguments given
above.
-m68000-mc68000
Generate output for a 68000. This is the default when the compiler
is configured for 68000-based systems. It is equivalent to
-march=68000.
Use this option for microcontrollers with a 68000 or EC000 core,
including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
-m68010
Generate output for a 68010. This is the default when the compiler
is configured for 68010-based systems. It is equivalent to
-march=68010.
-m68020-mc68020
Generate output for a 68020. This is the default when the compiler
is configured for 68020-based systems. It is equivalent to
-march=68020.
-m68030
Generate output for a 68030. This is the default when the compiler
is configured for 68030-based systems. It is equivalent to
-march=68030.
-m68040
Generate output for a 68040. This is the default when the compiler
is configured for 68040-based systems. It is equivalent to
-march=68040.
This option inhibits the use of 68881/68882 instructions that have
to be emulated by software on the 68040. Use this option if your
68040 does not have code to emulate those instructions.
-m68060
Generate output for a 68060. This is the default when the compiler
is configured for 68060-based systems. It is equivalent to
-march=68060.
This option inhibits the use of 68020 and 68881/68882 instructions
that have to be emulated by software on the 68060. Use this option
if your 68060 does not have code to emulate those instructions.
-mcpu32
Generate output for a CPU32. This is the default when the compiler
is configured for CPU32-based systems. It is equivalent to
-march=cpu32.
Use this option for microcontrollers with a CPU32 or CPU32+ core,
including the 68330, 68331, 68332, 68333, 68334, 68336, 68340,
68341, 68349 and 68360.
-m5200
Generate output for a 520X ColdFire CPU. This is the default when
the compiler is configured for 520X-based systems. It is
equivalent to -mcpu=5206, and is now deprecated in favor of that
option.
Use this option for microcontroller with a 5200 core, including the
MCF5202, MCF5203, MCF5204 and MCF5206.
-m5206e
Generate output for a 5206e ColdFire CPU. The option is now
deprecated in favor of the equivalent -mcpu=5206e.
-m528x
Generate output for a member of the ColdFire 528X family. The
option is now deprecated in favor of the equivalent -mcpu=528x.
-m5307
Generate output for a ColdFire 5307 CPU. The option is now
deprecated in favor of the equivalent -mcpu=5307.
-m5407
Generate output for a ColdFire 5407 CPU. The option is now
deprecated in favor of the equivalent -mcpu=5407.
-mcfv4e
Generate output for a ColdFire V4e family CPU (e.g. 547x/548x).
This includes use of hardware floating-point instructions. The
option is equivalent to -mcpu=547x, and is now deprecated in favor
of that option.
-m68020-40
Generate output for a 68040, without using any of the new
instructions. This results in code that can run relatively
efficiently on either a 68020/68881 or a 68030 or a 68040. The
generated code does use the 68881 instructions that are emulated on
the 68040.
The option is equivalent to -march=68020-mtune=68020-40.
-m68020-60
Generate output for a 68060, without using any of the new
instructions. This results in code that can run relatively
efficiently on either a 68020/68881 or a 68030 or a 68040. The
generated code does use the 68881 instructions that are emulated on
the 68060.
The option is equivalent to -march=68020-mtune=68020-60.
-mhard-float-m68881
Generate floating-point instructions. This is the default for
68020 and above, and for ColdFire devices that have an FPU. It
defines the macro __HAVE_68881__ on M680x0 targets and __mcffpu__
on ColdFire targets.
-msoft-float
Do not generate floating-point instructions; use library calls
instead. This is the default for 68000, 68010, and 68832 targets.
It is also the default for ColdFire devices that have no FPU.
-mdiv-mno-div
Generate (do not generate) ColdFire hardware divide and remainder
instructions. If -march is used without -mcpu, the default is "on"
for ColdFire architectures and "off" for M680x0 architectures.
Otherwise, the default is taken from the target CPU (either the
default CPU, or the one specified by -mcpu). For example, the
default is "off" for -mcpu=5206 and "on" for -mcpu=5206e.
gcc defines the macro __mcfhwdiv__ when this option is enabled.
-mshort
Consider type "int" to be 16 bits wide, like "short int".
Additionally, parameters passed on the stack are also aligned to a
16-bit boundary even on targets whose API mandates promotion to
32-bit.
-mno-short
Do not consider type "int" to be 16 bits wide. This is the
default.
-mnobitfield-mno-bitfield
Do not use the bit-field instructions. The -m68000, -mcpu32 and
-m5200 options imply -mnobitfield.
-mbitfield
Do use the bit-field instructions. The -m68020 option implies
-mbitfield. This is the default if you use a configuration
designed for a 68020.
-mrtd
Use a different function-calling convention, in which functions
that take a fixed number of arguments return with the "rtd"
instruction, which pops their arguments while returning. This
saves one instruction in the caller since there is no need to pop
the arguments there.
This calling convention is incompatible with the one normally used
on Unix, so you cannot use it if you need to call libraries
compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including "printf"); otherwise
incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020, 68030,
68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
-mno-rtd
Do not use the calling conventions selected by -mrtd. This is the
default.
-malign-int-mno-align-int
Control whether GCC aligns "int", "long", "long long", "float",
"double", and "long double" variables on a 32-bit boundary
(-malign-int) or a 16-bit boundary (-mno-align-int). Aligning
variables on 32-bit boundaries produces code that runs somewhat
faster on processors with 32-bit busses at the expense of more
memory.
Warning: if you use the -malign-int switch, GCC will align
structures containing the above types differently than most
published application binary interface specifications for the m68k.
-mpcrel
Use the pc-relative addressing mode of the 68000 directly, instead
of using a global offset table. At present, this option implies
-fpic, allowing at most a 16-bit offset for pc-relative addressing.
-fPIC is not presently supported with -mpcrel, though this could be
supported for 68020 and higher processors.
-mno-strict-align-mstrict-align
Do not (do) assume that unaligned memory references will be handled
by the system.
-msep-data
Generate code that allows the data segment to be located in a
different area of memory from the text segment. This allows for
execute in place in an environment without virtual memory
management. This option implies -fPIC.
-mno-sep-data
Generate code that assumes that the data segment follows the text
segment. This is the default.
-mid-shared-library
Generate code that supports shared libraries via the library ID
method. This allows for execute in place and shared libraries in
an environment without virtual memory management. This option
implies -fPIC.
-mno-id-shared-library
Generate code that doesn't assume ID based shared libraries are
being used. This is the default.
-mshared-library-id=n
Specified the identification number of the ID based shared library
being compiled. Specifying a value of 0 will generate more compact
code, specifying other values will force the allocation of that
number to the current library but is no more space or time
efficient than omitting this option.
-mxgot-mno-xgot
When generating position-independent code for ColdFire, generate
code that works if the GOT has more than 8192 entries. This code
is larger and slower than code generated without this option. On
M680x0 processors, this option is not needed; -fPIC suffices.
GCC normally uses a single instruction to load values from the GOT.
While this is relatively efficient, it only works if the GOT is
smaller than about 64k. Anything larger causes the linker to
report an error such as:
relocation truncated to fit: R_68K_GOT16O foobar
If this happens, you should recompile your code with -mxgot. It
should then work with very large GOTs. However, code generated
with -mxgot is less efficient, since it takes 4 instructions to
fetch the value of a global symbol.
Note that some linkers, including newer versions of the GNU linker,
can create multiple GOTs and sort GOT entries. If you have such a
linker, you should only need to use -mxgot when compiling a single
object file that accesses more than 8192 GOT entries. Very few do.
These options have no effect unless GCC is generating position-
independent code.
MCoreOptions
These are the -m options defined for the Motorola M*Core processors.
-mhardlit-mno-hardlit
Inline constants into the code stream if it can be done in two
instructions or less.
-mdiv-mno-div
Use the divide instruction. (Enabled by default).
-mrelax-immediate-mno-relax-immediate
Allow arbitrary sized immediates in bit operations.
-mwide-bitfields-mno-wide-bitfields
Always treat bit-fields as int-sized.
-m4byte-functions-mno-4byte-functions
Force all functions to be aligned to a 4-byte boundary.
-mcallgraph-data-mno-callgraph-data
Emit callgraph information.
-mslow-bytes-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian-mbig-endian
Generate code for a little-endian target.
-m210-m340
Generate code for the 210 processor.
-mno-lsim
Assume that runtime support has been provided and so omit the
simulator library (libsim.a) from the linker command line.
-mstack-increment=size
Set the maximum amount for a single stack increment operation.
Large values can increase the speed of programs that contain
functions that need a large amount of stack space, but they can
also trigger a segmentation fault if the stack is extended too
much. The default value is 0x1000.
MePOptions-mabsdiff
Enables the "abs" instruction, which is the absolute difference
between two registers.
-mall-opts
Enables all the optional instructions - average, multiply, divide,
bit operations, leading zero, absolute difference, min/max, clip,
and saturation.
-maverage
Enables the "ave" instruction, which computes the average of two
registers.
-mbased=n
Variables of size n bytes or smaller will be placed in the ".based"
section by default. Based variables use the $tp register as a base
register, and there is a 128-byte limit to the ".based" section.
-mbitops
Enables the bit operation instructions - bit test ("btstm"), set
("bsetm"), clear ("bclrm"), invert ("bnotm"), and test-and-set
("tas").
-mc=name
Selects which section constant data will be placed in. name may be
"tiny", "near", or "far".
-mclip
Enables the "clip" instruction. Note that "-mclip" is not useful
unless you also provide "-mminmax".
-mconfig=name
Selects one of the build-in core configurations. Each MeP chip has
one or more modules in it; each module has a core CPU and a variety
of coprocessors, optional instructions, and peripherals. The
"MeP-Integrator" tool, not part of GCC, provides these
configurations through this option; using this option is the same
as using all the corresponding command-line options. The default
configuration is "default".
-mcop
Enables the coprocessor instructions. By default, this is a 32-bit
coprocessor. Note that the coprocessor is normally enabled via the
"-mconfig=" option.
-mcop32
Enables the 32-bit coprocessor's instructions.
-mcop64
Enables the 64-bit coprocessor's instructions.
-mivc2
Enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor.
-mdc
Causes constant variables to be placed in the ".near" section.
-mdiv
Enables the "div" and "divu" instructions.
-meb
Generate big-endian code.
-mel
Generate little-endian code.
-mio-volatile
Tells the compiler that any variable marked with the "io" attribute
is to be considered volatile.
-ml Causes variables to be assigned to the ".far" section by default.
-mleadz
Enables the "leadz" (leading zero) instruction.
-mm Causes variables to be assigned to the ".near" section by default.
-mminmax
Enables the "min" and "max" instructions.
-mmult
Enables the multiplication and multiply-accumulate instructions.
-mno-opts
Disables all the optional instructions enabled by "-mall-opts".
-mrepeat
Enables the "repeat" and "erepeat" instructions, used for low-
overhead looping.
-ms Causes all variables to default to the ".tiny" section. Note that
there is a 65536-byte limit to this section. Accesses to these
variables use the %gp base register.
-msatur
Enables the saturation instructions. Note that the compiler does
not currently generate these itself, but this option is included
for compatibility with other tools, like "as".
-msdram
Link the SDRAM-based runtime instead of the default ROM-based
runtime.
-msim
Link the simulator runtime libraries.
-msimnovec
Link the simulator runtime libraries, excluding built-in support
for reset and exception vectors and tables.
-mtf
Causes all functions to default to the ".far" section. Without
this option, functions default to the ".near" section.
-mtiny=n
Variables that are n bytes or smaller will be allocated to the
".tiny" section. These variables use the $gp base register. The
default for this option is 4, but note that there's a 65536-byte
limit to the ".tiny" section.
MicroBlazeOptions-msoft-float
Use software emulation for floating point (default).
-mhard-float
Use hardware floating-point instructions.
-mmemcpy
Do not optimize block moves, use "memcpy".
-mno-clearbss
This option is deprecated. Use -fno-zero-initialized-in-bss
instead.
-mcpu=cpu-type
Use features of and schedule code for given CPU. Supported values
are in the format vX.YY.Z, where X is a major version, YY is the
minor version, and Z is compatibility code. Example values are
v3.00.a, v4.00.b, v5.00.a, v5.00.b, v5.00.b, v6.00.a.
-mxl-soft-mul
Use software multiply emulation (default).
-mxl-soft-div
Use software emulation for divides (default).
-mxl-barrel-shift
Use the hardware barrel shifter.
-mxl-pattern-compare
Use pattern compare instructions.
-msmall-divides
Use table lookup optimization for small signed integer divisions.
-mxl-stack-check
This option is deprecated. Use -fstack-check instead.
-mxl-gp-opt
Use GP relative sdata/sbss sections.
-mxl-multiply-high
Use multiply high instructions for high part of 32x32 multiply.
-mxl-float-convert
Use hardware floating-point conversion instructions.
-mxl-float-sqrt
Use hardware floating-point square root instruction.
-mxl-mode-app-model
Select application model app-model. Valid models are
executable
normal executable (default), uses startup code crt0.o.
xmdstub
for use with Xilinx Microprocessor Debugger (XMD) based
software intrusive debug agent called xmdstub. This uses
startup file crt1.o and sets the start address of the program
to be 0x800.
bootstrap
for applications that are loaded using a bootloader. This
model uses startup file crt2.o which does not contain a
processor reset vector handler. This is suitable for
transferring control on a processor reset to the bootloader
rather than the application.
novectors
for applications that do not require any of the MicroBlaze
vectors. This option may be useful for applications running
within a monitoring application. This model uses crt3.o as a
startup file.
Option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model.
MIPSOptions-EB Generate big-endian code.
-EL Generate little-endian code. This is the default for mips*el-*-*
configurations.
-march=arch
Generate code that will run on arch, which can be the name of a
generic MIPS ISA, or the name of a particular processor. The ISA
names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips64 and
mips64r2. The processor names are: 4kc, 4km, 4kp, 4ksc, 4kec,
4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec,
24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 74kc, 74kf2_1, 74kf1_1,
74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, loongson2e, loongson2f,
loongson3a, m4k, octeon, octeon+, octeon2, orion, r2000, r3000,
r3900, r4000, r4400, r4600, r4650, r6000, r8000, rm7000, rm9000,
r10000, r12000, r14000, r16000, sb1, sr71000, vr4100, vr4111,
vr4120, vr4130, vr4300, vr5000, vr5400, vr5500 and xlr. The
special value from-abi selects the most compatible architecture for
the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for
64-bit ABIs).
Native Linux/GNU and IRIX toolchains also support the value native,
which selects the best architecture option for the host processor.
-march=native has no effect if GCC does not recognize the
processor.
In processor names, a final 000 can be abbreviated as k (for
example, -march=r2k). Prefixes are optional, and vr may be written
r.
Names of the form nf2_1 refer to processors with FPUs clocked at
half the rate of the core, names of the form nf1_1 refer to
processors with FPUs clocked at the same rate as the core, and
names of the form nf3_2 refer to processors with FPUs clocked a
ratio of 3:2 with respect to the core. For compatibility reasons,
nf is accepted as a synonym for nf2_1 while nx and bfx are accepted
as synonyms for nf1_1.
GCC defines two macros based on the value of this option. The
first is _MIPS_ARCH, which gives the name of target architecture,
as a string. The second has the form _MIPS_ARCH_foo, where foo is
the capitalized value of _MIPS_ARCH. For example, -march=r2000
will set _MIPS_ARCH to "r2000" and define the macro
_MIPS_ARCH_R2000.
Note that the _MIPS_ARCH macro uses the processor names given
above. In other words, it will have the full prefix and will not
abbreviate 000 as k. In the case of from-abi, the macro names the
resolved architecture (either "mips1" or "mips3"). It names the
default architecture when no -march option is given.
-mtune=arch
Optimize for arch. Among other things, this option controls the
way instructions are scheduled, and the perceived cost of
arithmetic operations. The list of arch values is the same as for
-march.
When this option is not used, GCC will optimize for the processor
specified by -march. By using -march and -mtune together, it is
possible to generate code that will run on a family of processors,
but optimize the code for one particular member of that family.
-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work
in the same way as the -march ones described above.
-mips1
Equivalent to -march=mips1.
-mips2
Equivalent to -march=mips2.
-mips3
Equivalent to -march=mips3.
-mips4
Equivalent to -march=mips4.
-mips32
Equivalent to -march=mips32.
-mips32r2
Equivalent to -march=mips32r2.
-mips64
Equivalent to -march=mips64.
-mips64r2
Equivalent to -march=mips64r2.
-mips16-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targetting a
MIPS32 or MIPS64 architecture, it will make use of the MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-function
basis by means of "mips16" and "nomips16" attributes.
-mflip-mips16
Generate MIPS16 code on alternating functions. This option is
provided for regression testing of mixed MIPS16/non-MIPS16 code
generation, and is not intended for ordinary use in compiling user
code.
-minterlink-mips16-mno-interlink-mips16
Require (do not require) that non-MIPS16 code be link-compatible
with MIPS16 code.
For example, non-MIPS16 code cannot jump directly to MIPS16 code;
it must either use a call or an indirect jump. -minterlink-mips16
therefore disables direct jumps unless GCC knows that the target of
the jump is not MIPS16.
-mabi=32-mabi=o64-mabi=n32-mabi=64-mabi=eabi
Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally
generates 64-bit code when you select a 64-bit architecture, but
you can use -mgp32 to get 32-bit code instead.
For information about the O64 ABI, see
<http://gcc.gnu.org/projects/mipso64-abi.html>.
GCC supports a variant of the o32 ABI in which floating-point
registers are 64 rather than 32 bits wide. You can select this
combination with -mabi=32-mfp64. This ABI relies on the mthc1 and
mfhc1 instructions and is therefore only supported for MIPS32R2
processors.
The register assignments for arguments and return values remain the
same, but each scalar value is passed in a single 64-bit register
rather than a pair of 32-bit registers. For example, scalar
floating-point values are returned in $f0 only, not a $f0/$f1 pair.
The set of call-saved registers also remains the same, but all 64
bits are saved.
-mabicalls-mno-abicalls
Generate (do not generate) code that is suitable for SVR4-style
dynamic objects. -mabicalls is the default for SVR4-based systems.
-mshared-mno-shared
Generate (do not generate) code that is fully position-independent,
and that can therefore be linked into shared libraries. This
option only affects -mabicalls.
All -mabicalls code has traditionally been position-independent,
regardless of options like -fPIC and -fpic. However, as an
extension, the GNU toolchain allows executables to use absolute
accesses for locally-binding symbols. It can also use shorter GP
initialization sequences and generate direct calls to locally-
defined functions. This mode is selected by -mno-shared.
-mno-shared depends on binutils 2.16 or higher and generates
objects that can only be linked by the GNU linker. However, the
option does not affect the ABI of the final executable; it only
affects the ABI of relocatable objects. Using -mno-shared will
generally make executables both smaller and quicker.
-mshared is the default.
-mplt-mno-plt
Assume (do not assume) that the static and dynamic linkers support
PLTs and copy relocations. This option only affects -mno-shared-mabicalls. For the n64 ABI, this option has no effect without
-msym32.
You can make -mplt the default by configuring GCC with
--with-mips-plt. The default is -mno-plt otherwise.
-mxgot-mno-xgot
Lift (do not lift) the usual restrictions on the size of the global
offset table.
GCC normally uses a single instruction to load values from the GOT.
While this is relatively efficient, it will only work if the GOT is
smaller than about 64k. Anything larger will cause the linker to
report an error such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with -mxgot. It
should then work with very large GOTs, although it will also be
less efficient, since it will take three instructions to fetch the
value of a global symbol.
Note that some linkers can create multiple GOTs. If you have such
a linker, you should only need to use -mxgot when a single object
file accesses more than 64k's worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position
independent code.
-mgp32
Assume that general-purpose registers are 32 bits wide.
-mgp64
Assume that general-purpose registers are 64 bits wide.
-mfp32
Assume that floating-point registers are 32 bits wide.
-mfp64
Assume that floating-point registers are 64 bits wide.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions. Implement
floating-point calculations using library calls instead.
-msingle-float
Assume that the floating-point coprocessor only supports single-
precision operations.
-mdouble-float
Assume that the floating-point coprocessor supports double-
precision operations. This is the default.
-mllsc-mno-llsc
Use (do not use) ll, sc, and sync instructions to implement atomic
memory built-in functions. When neither option is specified, GCC
will use the instructions if the target architecture supports them.
-mllsc is useful if the runtime environment can emulate the
instructions and -mno-llsc can be useful when compiling for
nonstandard ISAs. You can make either option the default by
configuring GCC with --with-llsc and --without-llsc respectively.
--with-llsc is the default for some configurations; see the
installation documentation for details.
-mdsp-mno-dsp
Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro __mips_dsp. It also
defines __mips_dsp_rev to 1.
-mdspr2-mno-dspr2
Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros __mips_dsp and
__mips_dspr2. It also defines __mips_dsp_rev to 2.
-msmartmips-mno-smartmips
Use (do not use) the MIPS SmartMIPS ASE.
-mpaired-single-mno-paired-single
Use (do not use) paired-single floating-point instructions.
This option requires hardware floating-point support to be
enabled.
-mdmx-mno-mdmx
Use (do not use) MIPS Digital Media Extension instructions. This
option can only be used when generating 64-bit code and requires
hardware floating-point support to be enabled.
-mips3d-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d implies
-mpaired-single.
-mmt-mno-mt
Use (do not use) MT Multithreading instructions.
-mlong64
Force "long" types to be 64 bits wide. See -mlong32 for an
explanation of the default and the way that the pointer size is
determined.
-mlong32
Force "long", "int", and pointer types to be 32 bits wide.
The default size of "int"s, "long"s and pointers depends on the
ABI. All the supported ABIs use 32-bit "int"s. The n64 ABI uses
64-bit "long"s, as does the 64-bit EABI; the others use 32-bit
"long"s. Pointers are the same size as "long"s, or the same size
as integer registers, whichever is smaller.
-msym32-mno-sym32
Assume (do not assume) that all symbols have 32-bit values,
regardless of the selected ABI. This option is useful in
combination with -mabi=64 and -mno-abicalls because it allows GCC
to generate shorter and faster references to symbolic addresses.
-Gnum
Put definitions of externally-visible data in a small data section
if that data is no bigger than num bytes. GCC can then access the
data more efficiently; see -mgpopt for details.
The default -G option depends on the configuration.
-mlocal-sdata-mno-local-sdata
Extend (do not extend) the -G behavior to local data too, such as
to static variables in C. -mlocal-sdata is the default for all
configurations.
If the linker complains that an application is using too much small
data, you might want to try rebuilding the less performance-
critical parts with -mno-local-sdata. You might also want to build
large libraries with -mno-local-sdata, so that the libraries leave
more room for the main program.
-mextern-sdata-mno-extern-sdata
Assume (do not assume) that externally-defined data will be in a
small data section if that data is within the -G limit.
-mextern-sdata is the default for all configurations.
If you compile a module Mod with -mextern-sdata-Gnum-mgpopt, and
Mod references a variable Var that is no bigger than num bytes, you
must make sure that Var is placed in a small data section. If Var
is defined by another module, you must either compile that module
with a high-enough -G setting or attach a "section" attribute to
Var's definition. If Var is common, you must link the application
with a high-enough -G setting.
The easiest way of satisfying these restrictions is to compile and
link every module with the same -G option. However, you may wish
to build a library that supports several different small data
limits. You can do this by compiling the library with the highest
supported -G setting and additionally using -mno-extern-sdata to
stop the library from making assumptions about externally-defined
data.
-mgpopt-mno-gpopt
Use (do not use) GP-relative accesses for symbols that are known to
be in a small data section; see -G, -mlocal-sdata and
-mextern-sdata. -mgpopt is the default for all configurations.
-mno-gpopt is useful for cases where the $gp register might not
hold the value of "_gp". For example, if the code is part of a
library that might be used in a boot monitor, programs that call
boot monitor routines will pass an unknown value in $gp. (In such
situations, the boot monitor itself would usually be compiled with
-G0.)
-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.
-membedded-data-mno-embedded-data
Allocate variables to the read-only data section first if possible,
then next in the small data section if possible, otherwise in data.
This gives slightly slower code than the default, but reduces the
amount of RAM required when executing, and thus may be preferred
for some embedded systems.
-muninit-const-in-rodata-mno-uninit-const-in-rodata
Put uninitialized "const" variables in the read-only data section.
This option is only meaningful in conjunction with -membedded-data.
-mcode-readable=setting
Specify whether GCC may generate code that reads from executable
sections. There are three possible settings:
-mcode-readable=yes
Instructions may freely access executable sections. This is
the default setting.
-mcode-readable=pcrel
MIPS16 PC-relative load instructions can access executable
sections, but other instructions must not do so. This option
is useful on 4KSc and 4KSd processors when the code TLBs have
the Read Inhibit bit set. It is also useful on processors that
can be configured to have a dual instruction/data SRAM
interface and that, like the M4K, automatically redirect PC-
relative loads to the instruction RAM.
-mcode-readable=no
Instructions must not access executable sections. This option
can be useful on targets that are configured to have a dual
instruction/data SRAM interface but that (unlike the M4K) do
not automatically redirect PC-relative loads to the instruction
RAM.
-msplit-addresses-mno-split-addresses
Enable (disable) use of the "%hi()" and "%lo()" assembler
relocation operators. This option has been superseded by
-mexplicit-relocs but is retained for backwards compatibility.
-mexplicit-relocs-mno-explicit-relocs
Use (do not use) assembler relocation operators when dealing with
symbolic addresses. The alternative, selected by
-mno-explicit-relocs, is to use assembler macros instead.
-mexplicit-relocs is the default if GCC was configured to use an
assembler that supports relocation operators.
-mcheck-zero-division-mno-check-zero-division
Trap (do not trap) on integer division by zero.
The default is -mcheck-zero-division.
-mdivide-traps-mdivide-breaks
MIPS systems check for division by zero by generating either a
conditional trap or a break instruction. Using traps results in
smaller code, but is only supported on MIPS II and later. Also,
some versions of the Linux kernel have a bug that prevents trap
from generating the proper signal ("SIGFPE"). Use -mdivide-traps
to allow conditional traps on architectures that support them and
-mdivide-breaks to force the use of breaks.
The default is usually -mdivide-traps, but this can be overridden
at configure time using --with-divide=breaks. Divide-by-zero
checks can be completely disabled using -mno-check-zero-division.
-mmemcpy-mno-memcpy
Force (do not force) the use of "memcpy()" for non-trivial block
moves. The default is -mno-memcpy, which allows GCC to inline most
constant-sized copies.
-mlong-calls-mno-long-calls
Disable (do not disable) use of the "jal" instruction. Calling
functions using "jal" is more efficient but requires the caller and
callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is
-mno-long-calls.
-mmad-mno-mad
Enable (disable) use of the "mad", "madu" and "mul" instructions,
as provided by the R4650 ISA.
-mfused-madd-mno-fused-madd
Enable (disable) use of the floating-point multiply-accumulate
instructions, when they are available. The default is
-mfused-madd.
When multiply-accumulate instructions are used, the intermediate
product is calculated to infinite precision and is not subject to
the FCSR Flush to Zero bit. This may be undesirable in some
circumstances.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user
assembler files (with a .s suffix) when assembling them.
-mfix-24k-mno-fix-24k
Work around the 24K E48 (lost data on stores during refill) errata.
The workarounds are implemented by the assembler rather than by
GCC.
-mfix-r4000-mno-fix-r4000
Work around certain R4000 CPU errata:
- A double-word or a variable shift may give an incorrect result
if executed immediately after starting an integer division.
- A double-word or a variable shift may give an incorrect result
if executed while an integer multiplication is in progress.
- An integer division may give an incorrect result if started in
a delay slot of a taken branch or a jump.
-mfix-r4400-mno-fix-r4400
Work around certain R4400 CPU errata:
- A double-word or a variable shift may give an incorrect result
if executed immediately after starting an integer division.
-mfix-r10000-mno-fix-r10000
Work around certain R10000 errata:
- "ll"/"sc" sequences may not behave atomically on revisions
prior to 3.0. They may deadlock on revisions 2.6 and earlier.
This option can only be used if the target architecture supports
branch-likely instructions. -mfix-r10000 is the default when
-march=r10000 is used; -mno-fix-r10000 is the default otherwise.
-mfix-vr4120-mno-fix-vr4120
Work around certain VR4120 errata:
- "dmultu" does not always produce the correct result.
- "div" and "ddiv" do not always produce the correct result if
one of the operands is negative.
The workarounds for the division errata rely on special functions
in libgcc.a. At present, these functions are only provided by the
"mips64vr*-elf" configurations.
Other VR4120 errata require a nop to be inserted between certain
pairs of instructions. These errata are handled by the assembler,
not by GCC itself.
-mfix-vr4130
Work around the VR4130 "mflo"/"mfhi" errata. The workarounds are
implemented by the assembler rather than by GCC, although GCC will
avoid using "mflo" and "mfhi" if the VR4130 "macc", "macchi",
"dmacc" and "dmacchi" instructions are available instead.
-mfix-sb1-mno-fix-sb1
Work around certain SB-1 CPU core errata. (This flag currently
works around the SB-1 revision 2 "F1" and "F2" floating-point
errata.)
-mr10k-cache-barrier=setting
Specify whether GCC should insert cache barriers to avoid the side-
effects of speculation on R10K processors.
In common with many processors, the R10K tries to predict the
outcome of a conditional branch and speculatively executes
instructions from the "taken" branch. It later aborts these
instructions if the predicted outcome was wrong. However, on the
R10K, even aborted instructions can have side effects.
This problem only affects kernel stores and, depending on the
system, kernel loads. As an example, a speculatively-executed
store may load the target memory into cache and mark the cache line
as dirty, even if the store itself is later aborted. If a DMA
operation writes to the same area of memory before the "dirty" line
is flushed, the cached data will overwrite the DMA-ed data. See
the R10K processor manual for a full description, including other
potential problems.
One workaround is to insert cache barrier instructions before every
memory access that might be speculatively executed and that might
have side effects even if aborted. -mr10k-cache-barrier=setting
controls GCC's implementation of this workaround. It assumes that
aborted accesses to any byte in the following regions will not have
side effects:
1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant
address.
It is the kernel's responsibility to ensure that speculative
accesses to these regions are indeed safe.
If the input program contains a function declaration such as:
void foo (void);
then the implementation of "foo" must allow "j foo" and "jal foo"
to be executed speculatively. GCC honors this restriction for
functions it compiles itself. It expects non-GCC functions (such
as hand-written assembly code) to do the same.
The option has three forms:
-mr10k-cache-barrier=load-store
Insert a cache barrier before a load or store that might be
speculatively executed and that might have side effects even if
aborted.
-mr10k-cache-barrier=store
Insert a cache barrier before a store that might be
speculatively executed and that might have side effects even if
aborted.
-mr10k-cache-barrier=none
Disable the insertion of cache barriers. This is the default
setting.
-mflush-func=func-mno-flush-func
Specifies the function to call to flush the I and D caches, or to
not call any such function. If called, the function must take the
same arguments as the common "_flush_func()", that is, the address
of the memory range for which the cache is being flushed, the size
of the memory range, and the number 3 (to flush both caches). The
default depends on the target GCC was configured for, but commonly
is either _flush_func or __cpu_flush.
mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions.
This cost is only a heuristic and is not guaranteed to produce
consistent results across releases. A zero cost redundantly
selects the default, which is based on the -mtune setting.
-mbranch-likely-mno-branch-likely
Enable or disable use of Branch Likely instructions, regardless of
the default for the selected architecture. By default, Branch
Likely instructions may be generated if they are supported by the
selected architecture. An exception is for the MIPS32 and MIPS64
architectures and processors that implement those architectures;
for those, Branch Likely instructions will not be generated by
default because the MIPS32 and MIPS64 architectures specifically
deprecate their use.
-mfp-exceptions-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how we
schedule FP instructions for some processors. The default is that
FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we
are emitting 64-bit code, then we can use both FP pipes.
Otherwise, we can only use one FP pipe.
-mvr4130-align-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only issue two
instructions together if the first one is 8-byte aligned. When
this option is enabled, GCC will align pairs of instructions that
it thinks should execute in parallel.
This option only has an effect when optimizing for the VR4130. It
normally makes code faster, but at the expense of making it bigger.
It is enabled by default at optimization level -O3.
-msynci-mno-synci
Enable (disable) generation of "synci" instructions on
architectures that support it. The "synci" instructions (if
enabled) will be generated when "__builtin___clear_cache()" is
compiled.
This option defaults to "-mno-synci", but the default can be
overridden by configuring with "--with-synci".
When compiling code for single processor systems, it is generally
safe to use "synci". However, on many multi-core (SMP) systems, it
will not invalidate the instruction caches on all cores and may
lead to undefined behavior.
-mrelax-pic-calls-mno-relax-pic-calls
Try to turn PIC calls that are normally dispatched via register $25
into direct calls. This is only possible if the linker can resolve
the destination at link-time and if the destination is within range
for a direct call.
-mrelax-pic-calls is the default if GCC was configured to use an
assembler and a linker that supports the ".reloc" assembly
directive and "-mexplicit-relocs" is in effect. With
"-mno-explicit-relocs", this optimization can be performed by the
assembler and the linker alone without help from the compiler.
-mmcount-ra-address-mno-mcount-ra-address
Emit (do not emit) code that allows "_mcount" to modify the calling
function's return address. When enabled, this option extends the
usual "_mcount" interface with a new ra-address parameter, which
has type "intptr_t *" and is passed in register $12. "_mcount" can
then modify the return address by doing both of the following:
o Returning the new address in register $31.
o Storing the new address in "*ra-address", if ra-address is
nonnull.
The default is -mno-mcount-ra-address.
MMIXOptions
These options are defined for the MMIX:
-mlibfuncs-mno-libfuncs
Specify that intrinsic library functions are being compiled,
passing all values in registers, no matter the size.
-mepsilon-mno-epsilon
Generate floating-point comparison instructions that compare with
respect to the "rE" epsilon register.
-mabi=mmixware-mabi=gnu
Generate code that passes function parameters and return values
that (in the called function) are seen as registers $0 and up, as
opposed to the GNU ABI which uses global registers $231 and up.
-mzero-extend-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits, use
(do not use) zero-extending load instructions by default, rather
than sign-extending ones.
-mknuthdiv-mno-knuthdiv
Make the result of a division yielding a remainder have the same
sign as the divisor. With the default, -mno-knuthdiv, the sign of
the remainder follows the sign of the dividend. Both methods are
arithmetically valid, the latter being almost exclusively used.
-mtoplevel-symbols-mno-toplevel-symbols
Prepend (do not prepend) a : to all global symbols, so the assembly
code can be used with the "PREFIX" assembly directive.
-melf
Generate an executable in the ELF format, rather than the default
mmo format used by the mmix simulator.
-mbranch-predict-mno-branch-predict
Use (do not use) the probable-branch instructions, when static
branch prediction indicates a probable branch.
-mbase-addresses-mno-base-addresses
Generate (do not generate) code that uses baseaddresses. Using a
base address automatically generates a request (handled by the
assembler and the linker) for a constant to be set up in a global
register. The register is used for one or more base address
requests within the range 0 to 255 from the value held in the
register. The generally leads to short and fast code, but the
number of different data items that can be addressed is limited.
This means that a program that uses lots of static data may require
-mno-base-addresses.
-msingle-exit-mno-single-exit
Force (do not force) generated code to have a single exit point in
each function.
MN10300Options
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the
MN10300 processors. This is the default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions for
the MN10300 processors.
-mam33
Generate code using features specific to the AM33 processor.
-mno-am33
Do not generate code using features specific to the AM33 processor.
This is the default.
-mam33-2
Generate code using features specific to the AM33/2.0 processor.
-mam34
Generate code using features specific to the AM34 processor.
-mtune=cpu-type
Use the timing characteristics of the indicated CPU type when
scheduling instructions. This does not change the targeted
processor type. The CPU type must be one of mn10300, am33, am33-2
or am34.
-mreturn-pointer-on-d0
When generating a function that returns a pointer, return the
pointer in both "a0" and "d0". Otherwise, the pointer is returned
only in a0, and attempts to call such functions without a prototype
would result in errors. Note that this option is on by default;
use -mno-return-pointer-on-d0 to disable it.
-mno-crt0
Do not link in the C run-time initialization object file.
-mrelax
Indicate to the linker that it should perform a relaxation
optimization pass to shorten branches, calls and absolute memory
addresses. This option only has an effect when used on the command
line for the final link step.
This option makes symbolic debugging impossible.
-mliw
Allow the compiler to generate LongInstructionWord instructions
if the target is the AM33 or later. This is the default. This
option defines the preprocessor macro __LIW__.
-mnoliw
Do not allow the compiler to generate LongInstructionWord
instructions. This option defines the preprocessor macro
__NO_LIW__.
-msetlb
Allow the compiler to generate the SETLB and Lcc instructions if
the target is the AM33 or later. This is the default. This option
defines the preprocessor macro __SETLB__.
-mnosetlb
Do not allow the compiler to generate SETLB or Lcc instructions.
This option defines the preprocessor macro __NO_SETLB__.
PDP-11Options
These options are defined for the PDP-11:
-mfpu
Use hardware FPP floating point. This is the default. (FIS
floating point on the PDP-11/40 is not supported.)
-msoft-float
Do not use hardware floating point.
-mac0
Return floating-point results in ac0 (fr0 in Unix assembler
syntax).
-mno-ac0
Return floating-point results in memory. This is the default.
-m40
Generate code for a PDP-11/40.
-m45
Generate code for a PDP-11/45. This is the default.
-m10
Generate code for a PDP-11/10.
-mbcopy-builtin
Use inline "movmemhi" patterns for copying memory. This is the
default.
-mbcopy
Do not use inline "movmemhi" patterns for copying memory.
-mint16-mno-int32
Use 16-bit "int". This is the default.
-mint32-mno-int16
Use 32-bit "int".
-mfloat64-mno-float32
Use 64-bit "float". This is the default.
-mfloat32-mno-float64
Use 32-bit "float".
-mabshi
Use "abshi2" pattern. This is the default.
-mno-abshi
Do not use "abshi2" pattern.
-mbranch-expensive
Pretend that branches are expensive. This is for experimenting
with code generation only.
-mbranch-cheap
Do not pretend that branches are expensive. This is the default.
-munix-asm
Use Unix assembler syntax. This is the default when configured for
pdp11-*-bsd.
-mdec-asm
Use DEC assembler syntax. This is the default when configured for
any PDP-11 target other than pdp11-*-bsd.
picoChipOptions
These -m options are defined for picoChip implementations:
-mae=ae_type
Set the instruction set, register set, and instruction scheduling
parameters for array element type ae_type. Supported values for
ae_type are ANY, MUL, and MAC.
-mae=ANY selects a completely generic AE type. Code generated with
this option will run on any of the other AE types. The code will
not be as efficient as it would be if compiled for a specific AE
type, and some types of operation (e.g., multiplication) will not
work properly on all types of AE.
-mae=MUL selects a MUL AE type. This is the most useful AE type
for compiled code, and is the default.
-mae=MAC selects a DSP-style MAC AE. Code compiled with this
option may suffer from poor performance of byte (char)
manipulation, since the DSP AE does not provide hardware support
for byte load/stores.
-msymbol-as-address
Enable the compiler to directly use a symbol name as an address in
a load/store instruction, without first loading it into a register.
Typically, the use of this option will generate larger programs,
which run faster than when the option isn't used. However, the
results vary from program to program, so it is left as a user
option, rather than being permanently enabled.
-mno-inefficient-warnings
Disables warnings about the generation of inefficient code. These
warnings can be generated, for example, when compiling code that
performs byte-level memory operations on the MAC AE type. The MAC
AE has no hardware support for byte-level memory operations, so all
byte load/stores must be synthesized from word load/store
operations. This is inefficient and a warning will be generated
indicating to the programmer that they should rewrite the code to
avoid byte operations, or to target an AE type that has the
necessary hardware support. This option enables the warning to be
turned off.
PowerPCOptions
These are listed under
RL78Options-msim
Links in additional target libraries to support operation within a
simulator.
-mmul=none-mmul=g13-mmul=rl78
Specifies the type of hardware multiplication support to be used.
The default is "none", which uses software multiplication
functions. The "g13" option is for the hardware multiply/divide
peripheral only on the RL78/G13 targets. The "rl78" option is for
the standard hardware multiplication defined in the RL78 software
manual.
IBMRS/6000andPowerPCOptions
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpower-mno-power-mpower2-mno-power2-mpowerpc-mno-powerpc-mpowerpc-gpopt-mno-powerpc-gpopt-mpowerpc-gfxopt-mno-powerpc-gfxopt-mpowerpc64-mno-powerpc64-mmfcrf-mno-mfcrf-mpopcntb-mno-popcntb-mpopcntd-mno-popcntd-mfprnd-mno-fprnd-mcmpb-mno-cmpb-mmfpgpr-mno-mfpgpr-mhard-dfp-mno-hard-dfp
GCC supports two related instruction set architectures for the
RS/6000 and PowerPC. The POWER instruction set are those
instructions supported by the rios chip set used in the original
RS/6000 systems and the PowerPC instruction set is the architecture
of the Freescale MPC5xx, MPC6xx, MPC8xx microprocessors, and the
IBM 4xx, 6xx, and follow-on microprocessors.
Neither architecture is a subset of the other. However there is a
large common subset of instructions supported by both. An MQ
register is included in processors supporting the POWER
architecture.
You use these options to specify which instructions are available
on the processor you are using. The default value of these options
is determined when configuring GCC. Specifying the -mcpu=cpu_type
overrides the specification of these options. We recommend you use
the -mcpu=cpu_type option rather than the options listed above.
The -mpower option allows GCC to generate instructions that are
found only in the POWER architecture and to use the MQ register.
Specifying -mpower2 implies -power and also allows GCC to generate
instructions that are present in the POWER2 architecture but not
the original POWER architecture.
The -mpowerpc option allows GCC to generate instructions that are
found only in the 32-bit subset of the PowerPC architecture.
Specifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to
use the optional PowerPC architecture instructions in the General
Purpose group, including floating-point square root. Specifying
-mpowerpc-gfxopt implies -mpowerpc and also allows GCC to use the
optional PowerPC architecture instructions in the Graphics group,
including floating-point select.
The -mmfcrf option allows GCC to generate the move from condition
register field instruction implemented on the POWER4 processor and
other processors that support the PowerPC V2.01 architecture. The
-mpopcntb option allows GCC to generate the popcount and double-
precision FP reciprocal estimate instruction implemented on the
POWER5 processor and other processors that support the PowerPC
V2.02 architecture. The -mpopcntd option allows GCC to generate
the popcount instruction implemented on the POWER7 processor and
other processors that support the PowerPC V2.06 architecture. The
-mfprnd option allows GCC to generate the FP round to integer
instructions implemented on the POWER5+ processor and other
processors that support the PowerPC V2.03 architecture. The -mcmpb
option allows GCC to generate the compare bytes instruction
implemented on the POWER6 processor and other processors that
support the PowerPC V2.05 architecture. The -mmfpgpr option allows
GCC to generate the FP move to/from general-purpose register
instructions implemented on the POWER6X processor and other
processors that support the extended PowerPC V2.05 architecture.
The -mhard-dfp option allows GCC to generate the decimal floating-
point instructions implemented on some POWER processors.
The -mpowerpc64 option allows GCC to generate the additional 64-bit
instructions that are found in the full PowerPC64 architecture and
to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
-mno-powerpc64.
If you specify both -mno-power and -mno-powerpc, GCC will use only
the instructions in the common subset of both architectures plus
some special AIX common-mode calls, and will not use the MQ
register. Specifying both -mpower and -mpowerpc permits GCC to use
any instruction from either architecture and to allow use of the MQ
register; specify this for the Motorola MPC601.
-mnew-mnemonics-mold-mnemonics
Select which mnemonics to use in the generated assembler code.
With -mnew-mnemonics, GCC uses the assembler mnemonics defined for
the PowerPC architecture. With -mold-mnemonics it uses the
assembler mnemonics defined for the POWER architecture.
Instructions defined in only one architecture have only one
mnemonic; GCC uses that mnemonic irrespective of which of these
options is specified.
GCC defaults to the mnemonics appropriate for the architecture in
use. Specifying -mcpu=cpu_type sometimes overrides the value of
these option. Unless you are building a cross-compiler, you should
normally not specify either -mnew-mnemonics or -mold-mnemonics, but
should instead accept the default.
-mcpu=cpu_type
Set architecture type, register usage, choice of mnemonics, and
instruction scheduling parameters for machine type cpu_type.
Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp,
464, 464fp, 476, 476fp, 505, 601, 602, 603, 603e, 604, 604e, 620,
630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2,
e300c2, e300c3, e500mc, e500mc64, ec603e, G3, G4, G5, titan, power,
power2, power3, power4, power5, power5+, power6, power6x, power7,
common, powerpc, powerpc64, rios, rios1, rios2, rsc, and rs64.
-mcpu=common selects a completely generic processor. Code
generated under this option will run on any POWER or PowerPC
processor. GCC will use only the instructions in the common subset
of both architectures, and will not use the MQ register. GCC
assumes a generic processor model for scheduling purposes.
-mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64
specify generic POWER, POWER2, pure 32-bit PowerPC (i.e., not
MPC601), and 64-bit PowerPC architecture machine types, with an
appropriate, generic processor model assumed for scheduling
purposes.
The other options specify a specific processor. Code generated
under those options will run best on that processor, and may not
run at all on others.
The -mcpu options automatically enable or disable the following
options:
-maltivec-mfprnd-mhard-float-mmfcrf-mmultiple-mnew-mnemonics-mpopcntb-mpopcntd-mpower-mpower2-mpowerpc64-mpowerpc-gpopt-mpowerpc-gfxopt-msingle-float-mdouble-float-msimple-fpu-mstring-mmulhw-mdlmzb-mmfpgpr-mvsx
The particular options set for any particular CPU will vary between
compiler versions, depending on what setting seems to produce
optimal code for that CPU; it doesn't necessarily reflect the
actual hardware's capabilities. If you wish to set an individual
option to a particular value, you may specify it after the -mcpu
option, like -mcpu=970-mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled or
disabled by the -mcpu option at present because AIX does not have
full support for these options. You may still enable or disable
them individually if you're sure it'll work in your environment.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the architecture type, register usage, or
choice of mnemonics, as -mcpu=cpu_type would. The same values for
cpu_type are used for -mtune as for -mcpu. If both are specified,
the code generated will use the architecture, registers, and
mnemonics set by -mcpu, but the scheduling parameters set by
-mtune.
-mcmodel=small
Generate PowerPC64 code for the small model: The TOC is limited to
64k.
-mcmodel=medium
Generate PowerPC64 code for the medium model: The TOC and other
static data may be up to a total of 4G in size.
-mcmodel=large
Generate PowerPC64 code for the large model: The TOC may be up to
4G in size. Other data and code is only limited by the 64-bit
address space.
-maltivec-mno-altivec
Generate code that uses (does not use) AltiVec instructions, and
also enable the use of built-in functions that allow more direct
access to the AltiVec instruction set. You may also need to set
-mabi=altivec to adjust the current ABI with AltiVec ABI
enhancements.
-mvrsave-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-mgen-cell-microcode
Generate Cell microcode instructions
-mwarn-cell-microcode
Warning when a Cell microcode instruction is going to emitted. An
example of a Cell microcode instruction is a variable shift.
-msecure-plt
Generate code that allows ld and ld.so to build executables and
shared libraries with non-exec .plt and .got sections. This is a
PowerPC 32-bit SYSV ABI option.
-mbss-plt
Generate code that uses a BSS .plt section that ld.so fills in, and
requires .plt and .got sections that are both writable and
executable. This is a PowerPC 32-bit SYSV ABI option.
-misel-mno-isel
This switch enables or disables the generation of ISEL
instructions.
-misel=yes/no
This switch has been deprecated. Use -misel and -mno-isel instead.
-mspe-mno-spe
This switch enables or disables the generation of SPE simd
instructions.
-mpaired-mno-paired
This switch enables or disables the generation of PAIRED simd
instructions.
-mspe=yes/no
This option has been deprecated. Use -mspe and -mno-spe instead.
-mvsx-mno-vsx
Generate code that uses (does not use) vector/scalar (VSX)
instructions, and also enable the use of built-in functions that
allow more direct access to the VSX instruction set.
-mfloat-gprs=yes/single/double/no-mfloat-gprs
This switch enables or disables the generation of floating-point
operations on the general-purpose registers for architectures that
support it.
The argument yes or single enables the use of single-precision
floating-point operations.
The argument double enables the use of single and double-precision
floating-point operations.
The argument no disables floating-point operations on the general-
purpose registers.
This option is currently only available on the MPC854x.
-m32-m64
Generate code for 32-bit or 64-bit environments of Darwin and SVR4
targets (including GNU/Linux). The 32-bit environment sets int,
long and pointer to 32 bits and generates code that runs on any
PowerPC variant. The 64-bit environment sets int to 32 bits and
long and pointer to 64 bits, and generates code for PowerPC64, as
for -mpowerpc64.
-mfull-toc-mno-fp-in-toc-mno-sum-in-toc-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created
for every executable file. The -mfull-toc option is selected by
default. In that case, GCC will allocate at least one TOC entry
for each unique non-automatic variable reference in your program.
GCC will also place floating-point constants in the TOC. However,
only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have
overflowed the available TOC space, you can reduce the amount of
TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options.
-mno-fp-in-toc prevents GCC from putting floating-point constants
in the TOC and -mno-sum-in-toc forces GCC to generate code to
calculate the sum of an address and a constant at run time instead
of putting that sum into the TOC. You may specify one or both of
these options. Each causes GCC to produce very slightly slower and
larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both
of these options, specify -mminimal-toc instead. This option
causes GCC to make only one TOC entry for every file. When you
specify this option, GCC will produce code that is slower and
larger but which uses extremely little TOC space. You may wish to
use this option only on files that contain less frequently executed
code.
-maix64-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers,
64-bit "long" type, and the infrastructure needed to support them.
Specifying -maix64 implies -mpowerpc64 and -mpowerpc, while -maix32
disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults
to -maix32.
-mxl-compat-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler
semantics when using AIX-compatible ABI. Pass floating-point
arguments to prototyped functions beyond the register save area
(RSA) on the stack in addition to argument FPRs. Do not assume
that most significant double in 128-bit long double value is
properly rounded when comparing values and converting to double.
Use XL symbol names for long double support routines.
The AIX calling convention was extended but not initially
documented to handle an obscure K&R C case of calling a function
that takes the address of its arguments with fewer arguments than
declared. IBM XL compilers access floating-point arguments that do
not fit in the RSA from the stack when a subroutine is compiled
without optimization. Because always storing floating-point
arguments on the stack is inefficient and rarely needed, this
option is not enabled by default and only is necessary when calling
subroutines compiled by IBM XL compilers without optimization.
-mpe
Support IBMRS/6000SPParallelEnvironment (PE). Link an
application written to use message passing with special startup
code to enable the application to run. The system must have PE
installed in the standard location (/usr/lpp/ppe.poe/), or the
specs file must be overridden with the -specs= option to specify
the appropriate directory location. The Parallel Environment does
not support threads, so the -mpe option and the -pthread option are
incompatible.
-malign-natural-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option
-malign-natural overrides the ABI-defined alignment of larger
types, such as floating-point doubles, on their natural size-based
boundary. The option -malign-power instructs GCC to follow the
ABI-specified alignment rules. GCC defaults to the standard
alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and
-malign-power is not supported.
-msoft-float-mhard-float
Generate code that does not use (uses) the floating-point register
set. Software floating-point emulation is provided if you use the
-msoft-float option, and pass the option to GCC when linking.
-msingle-float-mdouble-float
Generate code for single- or double-precision floating-point
operations. -mdouble-float implies -msingle-float.
-msimple-fpu
Do not generate sqrt and div instructions for hardware floating-
point unit.
-mfpu
Specify type of floating-point unit. Valid values are sp_lite
(equivalent to -msingle-float -msimple-fpu), dp_lite (equivalent to
-mdouble-float -msimple-fpu), sp_full (equivalent to
-msingle-float), and dp_full (equivalent to -mdouble-float).
-mxilinx-fpu
Perform optimizations for the floating-point unit on Xilinx PPC
405/440.
-mmultiple-mno-multiple
Generate code that uses (does not use) the load multiple word
instructions and the store multiple word instructions. These
instructions are generated by default on POWER systems, and not
generated on PowerPC systems. Do not use -mmultiple on little-
endian PowerPC systems, since those instructions do not work when
the processor is in little-endian mode. The exceptions are PPC740
and PPC750 which permit these instructions in little-endian mode.
-mstring-mno-string
Generate code that uses (does not use) the load string instructions
and the store string word instructions to save multiple registers
and do small block moves. These instructions are generated by
default on POWER systems, and not generated on PowerPC systems. Do
not use -mstring on little-endian PowerPC systems, since those
instructions do not work when the processor is in little-endian
mode. The exceptions are PPC740 and PPC750 which permit these
instructions in little-endian mode.
-mupdate-mno-update
Generate code that uses (does not use) the load or store
instructions that update the base register to the address of the
calculated memory location. These instructions are generated by
default. If you use -mno-update, there is a small window between
the time that the stack pointer is updated and the address of the
previous frame is stored, which means code that walks the stack
frame across interrupts or signals may get corrupted data.
-mavoid-indexed-addresses-mno-avoid-indexed-addresses
Generate code that tries to avoid (not avoid) the use of indexed
load or store instructions. These instructions can incur a
performance penalty on Power6 processors in certain situations,
such as when stepping through large arrays that cross a 16M
boundary. This option is enabled by default when targetting Power6
and disabled otherwise.
-mfused-madd-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply
and accumulate instructions. These instructions are generated by
default if hardware floating point is used. The machine-dependent
-mfused-madd option is now mapped to the machine-independent
-ffp-contract=fast option, and -mno-fused-madd is mapped to
-ffp-contract=off.
-mmulhw-mno-mulhw
Generate code that uses (does not use) the half-word multiply and
multiply-accumulate instructions on the IBM 405, 440, 464 and 476
processors. These instructions are generated by default when
targetting those processors.
-mdlmzb-mno-dlmzb
Generate code that uses (does not use) the string-search dlmzb
instruction on the IBM 405, 440, 464 and 476 processors. This
instruction is generated by default when targetting those
processors.
-mno-bit-align-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force
structures and unions that contain bit-fields to be aligned to the
base type of the bit-field.
For example, by default a structure containing nothing but 8
"unsigned" bit-fields of length 1 is aligned to a 4-byte boundary
and has a size of 4 bytes. By using -mno-bit-align, the structure
is aligned to a 1-byte boundary and is 1 byte in size.
-mno-strict-align-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that
unaligned memory references will be handled by the system.
-mrelocatable-mno-relocatable
Generate code that allows (does not allow) a static executable to
be relocated to a different address at run time. A simple embedded
PowerPC system loader should relocate the entire contents of
".got2" and 4-byte locations listed in the ".fixup" section, a
table of 32-bit addresses generated by this option. For this to
work, all objects linked together must be compiled with
-mrelocatable or -mrelocatable-lib. -mrelocatable code aligns the
stack to an 8-byte boundary.
-mrelocatable-lib-mno-relocatable-lib
Like -mrelocatable, -mrelocatable-lib generates a ".fixup" section
to allow static executables to be relocated at run time, but
-mrelocatable-lib does not use the smaller stack alignment of
-mrelocatable. Objects compiled with -mrelocatable-lib may be
linked with objects compiled with any combination of the
-mrelocatable options.
-mno-toc-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that
register 2 contains a pointer to a global area pointing to the
addresses used in the program.
-mlittle-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the
processor in little-endian mode. The -mlittle-endian option is the
same as -mlittle.
-mbig-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the
processor in big-endian mode. The -mbig-endian option is the same
as -mbig.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not
relocatable, but that its external references are relocatable. The
resulting code is suitable for applications, but not shared
libraries.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather
than loading it in the prologue for each function. The runtime
system is responsible for initializing this register with an
appropriate value before execution begins.
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to dispatch-slot
restricted instructions during the second scheduling pass. The
argument priority takes the value 0/1/2 to assign
no/highest/second-highest priority to dispatch slot restricted
instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered costly by the
target during instruction scheduling. The argument dependence_type
takes one of the following values: no: no dependence is costly,
all: all dependences are costly, true_store_to_load: a true
dependence from store to load is costly, store_to_load: any
dependence from store to load is costly, number: any dependence for
which latency >= number is costly.
-minsert-sched-nops=scheme
This option controls which nop insertion scheme will be used during
the second scheduling pass. The argument scheme takes one of the
following values: no: Don't insert nops. pad: Pad with nops any
dispatch group that has vacant issue slots, according to the
scheduler's grouping. regroup_exact: Insert nops to force costly
dependent insns into separate groups. Insert exactly as many nops
as needed to force an insn to a new group, according to the
estimated processor grouping. number: Insert nops to force costly
dependent insns into separate groups. Insert number nops to force
an insn to a new group.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using
calling conventions that adheres to the March 1995 draft of the
System V Application Binary Interface, PowerPC processor
supplement. This is the default unless you configured GCC using
powerpc-*-eabiaix.
-mcall-sysv-eabi-mcall-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aixdesc
On System V.4 and embedded PowerPC systems compile code for the AIX
operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the
Linux-based GNU system.
-mcall-freebsd
On System V.4 and embedded PowerPC systems compile code for the
FreeBSD operating system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the
NetBSD operating system.
-mcall-openbsd
On System V.4 and embedded PowerPC systems compile code for the
OpenBSD operating system.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified
by the SVR4 ABI).
-mabi=abi-type
Extend the current ABI with a particular extension, or remove such
extension. Valid values are altivec, no-altivec, spe, no-spe,
ibmlongdouble, ieeelongdouble.
-mabi=spe
Extend the current ABI with SPE ABI extensions. This does not
change the default ABI, instead it adds the SPE ABI extensions to
the current ABI.
-mabi=no-spe
Disable Booke SPE ABI extensions for the current ABI.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision long double.
This is a PowerPC 32-bit SYSV ABI option.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision long double.
This is a PowerPC 32-bit Linux ABI option.
-mprototype-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to
variable argument functions are properly prototyped. Otherwise,
the compiler must insert an instruction before every non prototyped
call to set or clear bit 6 of the condition code register (CR) to
indicate whether floating-point values were passed in the floating-
point registers in case the function takes variable arguments.
With -mprototype, only calls to prototyped variable argument
functions will set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is
called sim-crt0.o and that the standard C libraries are libsim.a
and libc.a. This is the default for powerpc-*-eabisim
configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are libmvme.a and
libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are libads.a and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are libyk.a and libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are
compiling for a VxWorks system.
-memb
On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags
header to indicate that eabi extended relocations are used.
-meabi-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to
the Embedded Applications Binary Interface (eabi) which is a set of
modifications to the System V.4 specifications. Selecting -meabi
means that the stack is aligned to an 8-byte boundary, a function
"__eabi" is called to from "main" to set up the eabi environment,
and the -msdata option can use both "r2" and "r13" to point to two
separate small data areas. Selecting -mno-eabi means that the
stack is aligned to a 16-byte boundary, do not call an
initialization function from "main", and the -msdata option will
only use "r13" to point to a single small data area. The -meabi
option is on by default if you configured GCC using one of the
powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized
"const" global and static data in the .sdata2 section, which is
pointed to by register "r2". Put small initialized non-"const"
global and static data in the .sdata section, which is pointed to
by register "r13". Put small uninitialized global and static data
in the .sbss section, which is adjacent to the .sdata section. The
-msdata=eabi option is incompatible with the -mrelocatable option.
The -msdata=eabi option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and
static data in the .sdata section, which is pointed to by register
"r13". Put small uninitialized global and static data in the .sbss
section, which is adjacent to the .sdata section. The -msdata=sysv
option is incompatible with the -mrelocatable option.
-msdata=default-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used,
compile code the same as -msdata=eabi, otherwise compile code the
same as -msdata=sysv.
-msdata=data
On System V.4 and embedded PowerPC systems, put small global data
in the .sdata section. Put small uninitialized global data in the
.sbss section. Do not use register "r13" to address small data
however. This is the default behavior unless other -msdata options
are used.
-msdata=none-mno-sdata
On embedded PowerPC systems, put all initialized global and static
data in the .data section, and all uninitialized data in the .bss
section.
-mblock-move-inline-limit=num
Inline all block moves (such as calls to "memcpy" or structure
copies) less than or equal to num bytes. The minimum value for num
is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The
default value is target-specific.
-Gnum
On embedded PowerPC systems, put global and static items less than
or equal to num bytes into the small data or bss sections instead
of the normal data or bss section. By default, num is 8. The -Gnum switch is also passed to the linker. All modules should be
compiled with the same -Gnum value.
-mregnames-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit
register names in the assembly language output using symbolic
forms.
-mlongcall-mno-longcall
By default assume that all calls are far away so that a longer more
expensive calling sequence is required. This is required for calls
further than 32 megabytes (33,554,432 bytes) from the current
location. A short call will be generated if the compiler knows the
call cannot be that far away. This setting can be overridden by
the "shortcall" function attribute, or by "#pragma longcall(0)".
Some linkers are capable of detecting out-of-range calls and
generating glue code on the fly. On these systems, long calls are
unnecessary and generate slower code. As of this writing, the AIX
linker can do this, as can the GNU linker for PowerPC/64. It is
planned to add this feature to the GNU linker for 32-bit PowerPC
systems as well.
On Darwin/PPC systems, "#pragma longcall" will generate "jbsr
callee, L42", plus a "branch island" (glue code). The two target
addresses represent the callee and the "branch island". The
Darwin/PPC linker will prefer the first address and generate a "bl
callee" if the PPC "bl" instruction will reach the callee directly;
otherwise, the linker will generate "bl L42" to call the "branch
island". The "branch island" is appended to the body of the
calling function; it computes the full 32-bit address of the callee
and jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler emit
to the glue for every direct call, and the Darwin linker decides
whether to use or discard it.
In the future, we may cause GCC to ignore all longcall
specifications when the linker is known to generate glue.
-mtls-markers-mno-tls-markers
Mark (do not mark) calls to "__tls_get_addr" with a relocation
specifying the function argument. The relocation allows ld to
reliably associate function call with argument setup instructions
for TLS optimization, which in turn allows gcc to better schedule
the sequence.
-pthread
Adds support for multithreading with the pthreads library. This
option sets flags for both the preprocessor and linker.
-mrecip-mno-recip
This option will enable GCC to use the reciprocal estimate and
reciprocal square root estimate instructions with additional
Newton-Raphson steps to increase precision instead of doing a
divide or square root and divide for floating-point arguments. You
should use the -ffast-math option when using -mrecip (or at least
-funsafe-math-optimizations, -finite-math-only, -freciprocal-math
and -fno-trapping-math). Note that while the throughput of the
sequence is generally higher than the throughput of the non-
reciprocal instruction, the precision of the sequence can be
decreased by up to 2 ulp (i.e. the inverse of 1.0 equals
0.99999994) for reciprocal square roots.
-mrecip=opt
This option allows to control which reciprocal estimate
instructions may be used. opt is a comma separated list of
options, which may be preceded by a "!" to invert the option:
"all": enable all estimate instructions, "default": enable the
default instructions, equivalent to -mrecip, "none": disable all
estimate instructions, equivalent to -mno-recip; "div": enable the
reciprocal approximation instructions for both single and double
precision; "divf": enable the single-precision reciprocal
approximation instructions; "divd": enable the double-precision
reciprocal approximation instructions; "rsqrt": enable the
reciprocal square root approximation instructions for both single
and double precision; "rsqrtf": enable the single-precision
reciprocal square root approximation instructions; "rsqrtd": enable
the double-precision reciprocal square root approximation
instructions;
So for example, -mrecip=all,!rsqrtd would enable the all of the
reciprocal estimate instructions, except for the "FRSQRTE",
"XSRSQRTEDP", and "XVRSQRTEDP" instructions which handle the
double-precision reciprocal square root calculations.
-mrecip-precision-mno-recip-precision
Assume (do not assume) that the reciprocal estimate instructions
provide higher-precision estimates than is mandated by the PowerPC
ABI. Selecting -mcpu=power6 or -mcpu=power7 automatically selects
-mrecip-precision. The double-precision square root estimate
instructions are not generated by default on low-precision
machines, since they do not provide an estimate that converges
after three steps.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an
external library. The only type supported at present is "mass",
which specifies to use IBM's Mathematical Acceleration Subsystem
(MASS) libraries for vectorizing intrinsics using external
libraries. GCC will currently emit calls to "acosd2", "acosf4",
"acoshd2", "acoshf4", "asind2", "asinf4", "asinhd2", "asinhf4",
"atan2d2", "atan2f4", "atand2", "atanf4", "atanhd2", "atanhf4",
"cbrtd2", "cbrtf4", "cosd2", "cosf4", "coshd2", "coshf4", "erfcd2",
"erfcf4", "erfd2", "erff4", "exp2d2", "exp2f4", "expd2", "expf4",
"expm1d2", "expm1f4", "hypotd2", "hypotf4", "lgammad2", "lgammaf4",
"log10d2", "log10f4", "log1pd2", "log1pf4", "log2d2", "log2f4",
"logd2", "logf4", "powd2", "powf4", "sind2", "sinf4", "sinhd2",
"sinhf4", "sqrtd2", "sqrtf4", "tand2", "tanf4", "tanhd2", and
"tanhf4" when generating code for power7. Both -ftree-vectorize
and -funsafe-math-optimizations have to be enabled. The MASS
libraries will have to be specified at link time.
-mfriz-mno-friz
Generate (do not generate) the "friz" instruction when the
-funsafe-math-optimizations option is used to optimize rounding of
floating-point values to 64-bit integer and back to floating point.
The "friz" instruction does not return the same value if the
floating-point number is too large to fit in an integer.
-mpointers-to-nested-functions-mno-pointers-to-nested-functions
Generate (do not generate) code to load up the static chain
register (r11) when calling through a pointer on AIX and 64-bit
Linux systems where a function pointer points to a 3-word
descriptor giving the function address, TOC value to be loaded in
register r2, and static chain value to be loaded in register r11.
The -mpointers-to-nested-functions is on by default. You will not
be able to call through pointers to nested functions or pointers to
functions compiled in other languages that use the static chain if
you use the -mno-pointers-to-nested-functions.
-msave-toc-indirect-mno-save-toc-indirect
Generate (do not generate) code to save the TOC value in the
reserved stack location in the function prologue if the function
calls through a pointer on AIX and 64-bit Linux systems. If the
TOC value is not saved in the prologue, it is saved just before the
call through the pointer. The -mno-save-toc-indirect option is the
default.
RXOptions
These command-line options are defined for RX targets:
-m64bit-doubles-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits
(-m32bit-doubles) in size. The default is -m32bit-doubles. Note
RX floating-point hardware only works on 32-bit values, which is
why the default is -m32bit-doubles.
-fpu-nofpu
Enables (-fpu) or disables (-nofpu) the use of RX floating-point
hardware. The default is enabled for the RX600 series and disabled
for the RX200 series.
Floating-point instructions will only be generated for 32-bit
floating-point values however, so if the -m64bit-doubles option is
in use then the FPU hardware will not be used for doubles.
Note If the -fpu option is enabled then -funsafe-math-optimizations
is also enabled automatically. This is because the RX FPU
instructions are themselves unsafe.
-mcpu=name
Selects the type of RX CPU to be targeted. Currently three types
are supported, the generic RX600 and RX200 series hardware and the
specific RX610 CPU. The default is RX600.
The only difference between RX600 and RX610 is that the RX610 does
not support the "MVTIPL" instruction.
The RX200 series does not have a hardware floating-point unit and
so -nofpu is enabled by default when this type is selected.
-mbig-endian-data-mlittle-endian-data
Store data (but not code) in the big-endian format. The default is
-mlittle-endian-data, i.e. to store data in the little-endian
format.
-msmall-data-limit=N
Specifies the maximum size in bytes of global and static variables
which can be placed into the small data area. Using the small data
area can lead to smaller and faster code, but the size of area is
limited and it is up to the programmer to ensure that the area does
not overflow. Also when the small data area is used one of the
RX's registers (usually "r13") is reserved for use pointing to this
area, so it is no longer available for use by the compiler. This
could result in slower and/or larger code if variables which once
could have been held in the reserved register are now pushed onto
the stack.
Note, common variables (variables that have not been initialized)
and constants are not placed into the small data area as they are
assigned to other sections in the output executable.
The default value is zero, which disables this feature. Note, this
feature is not enabled by default with higher optimization levels
(-O2 etc) because of the potentially detrimental effects of
reserving a register. It is up to the programmer to experiment and
discover whether this feature is of benefit to their program. See
the description of the -mpid option for a description of how the
actual register to hold the small data area pointer is chosen.
-msim-mno-sim
Use the simulator runtime. The default is to use the libgloss
board specific runtime.
-mas100-syntax-mno-as100-syntax
When generating assembler output use a syntax that is compatible
with Renesas's AS100 assembler. This syntax can also be handled by
the GAS assembler but it has some restrictions so generating it is
not the default option.
-mmax-constant-size=N
Specifies the maximum size, in bytes, of a constant that can be
used as an operand in a RX instruction. Although the RX
instruction set does allow constants of up to 4 bytes in length to
be used in instructions, a longer value equates to a longer
instruction. Thus in some circumstances it can be beneficial to
restrict the size of constants that are used in instructions.
Constants that are too big are instead placed into a constant pool
and referenced via register indirection.
The value N can be between 0 and 4. A value of 0 (the default) or
4 means that constants of any size are allowed.
-mrelax
Enable linker relaxation. Linker relaxation is a process whereby
the linker will attempt to reduce the size of a program by finding
shorter versions of various instructions. Disabled by default.
-mint-register=N
Specify the number of registers to reserve for fast interrupt
handler functions. The value N can be between 0 and 4. A value of
1 means that register "r13" will be reserved for the exclusive use
of fast interrupt handlers. A value of 2 reserves "r13" and "r12".
A value of 3 reserves "r13", "r12" and "r11", and a value of 4
reserves "r13" through "r10". A value of 0, the default, does not
reserve any registers.
-msave-acc-in-interrupts
Specifies that interrupt handler functions should preserve the
accumulator register. This is only necessary if normal code might
use the accumulator register, for example because it performs
64-bit multiplications. The default is to ignore the accumulator
as this makes the interrupt handlers faster.
-mpid-mno-pid
Enables the generation of position independent data. When enabled
any access to constant data will done via an offset from a base
address held in a register. This allows the location of constant
data to be determined at run time without requiring the executable
to be relocated, which is a benefit to embedded applications with
tight memory constraints. Data that can be modified is not
affected by this option.
Note, using this feature reserves a register, usually "r13", for
the constant data base address. This can result in slower and/or
larger code, especially in complicated functions.
The actual register chosen to hold the constant data base address
depends upon whether the -msmall-data-limit and/or the
-mint-register command-line options are enabled. Starting with
register "r13" and proceeding downwards, registers are allocated
first to satisfy the requirements of -mint-register, then -mpid and
finally -msmall-data-limit. Thus it is possible for the small data
area register to be "r8" if both -mint-register=4 and -mpid are
specified on the command line.
By default this feature is not enabled. The default can be
restored via the -mno-pid command-line option.
Note: The generic GCC command-line option -ffixed-reg has special
significance to the RX port when used with the "interrupt" function
attribute. This attribute indicates a function intended to process
fast interrupts. GCC will will ensure that it only uses the registers
"r10", "r11", "r12" and/or "r13" and only provided that the normal use
of the corresponding registers have been restricted via the -ffixed-reg
or -mint-register command-line options.
S/390andzSeriesOptions
These are the -m options defined for the S/390 and zSeries
architecture.
-mhard-float-msoft-float
Use (do not use) the hardware floating-point instructions and
registers for floating-point operations. When -msoft-float is
specified, functions in libgcc.a will be used to perform floating-
point operations. When -mhard-float is specified, the compiler
generates IEEE floating-point instructions. This is the default.
-mhard-dfp-mno-hard-dfp
Use (do not use) the hardware decimal-floating-point instructions
for decimal-floating-point operations. When -mno-hard-dfp is
specified, functions in libgcc.a will be used to perform decimal-
floating-point operations. When -mhard-dfp is specified, the
compiler generates decimal-floating-point hardware instructions.
This is the default for -march=z9-ec or higher.
-mlong-double-64-mlong-double-128
These switches control the size of "long double" type. A size of 64
bits makes the "long double" type equivalent to the "double" type.
This is the default.
-mbackchain-mno-backchain
Store (do not store) the address of the caller's frame as backchain
pointer into the callee's stack frame. A backchain may be needed
to allow debugging using tools that do not understand DWARF-2 call
frame information. When -mno-packed-stack is in effect, the
backchain pointer is stored at the bottom of the stack frame; when
-mpacked-stack is in effect, the backchain is placed into the
topmost word of the 96/160 byte register save area.
In general, code compiled with -mbackchain is call-compatible with
code compiled with -mmo-backchain; however, use of the backchain
for debugging purposes usually requires that the whole binary is
built with -mbackchain. Note that the combination of -mbackchain,
-mpacked-stack and -mhard-float is not supported. In order to
build a linux kernel use -msoft-float.
The default is to not maintain the backchain.
-mpacked-stack-mno-packed-stack
Use (do not use) the packed stack layout. When -mno-packed-stack
is specified, the compiler uses the all fields of the 96/160 byte
register save area only for their default purpose; unused fields
still take up stack space. When -mpacked-stack is specified,
register save slots are densely packed at the top of the register
save area; unused space is reused for other purposes, allowing for
more efficient use of the available stack space. However, when
-mbackchain is also in effect, the topmost word of the save area is
always used to store the backchain, and the return address register
is always saved two words below the backchain.
As long as the stack frame backchain is not used, code generated
with -mpacked-stack is call-compatible with code generated with
-mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for
S/390 or zSeries generated code that uses the stack frame backchain
at run time, not just for debugging purposes. Such code is not
call-compatible with code compiled with -mpacked-stack. Also, note
that the combination of -mbackchain, -mpacked-stack and
-mhard-float is not supported. In order to build a linux kernel
use -msoft-float.
The default is to not use the packed stack layout.
-msmall-exec-mno-small-exec
Generate (or do not generate) code using the "bras" instruction to
do subroutine calls. This only works reliably if the total
executable size does not exceed 64k. The default is to use the
"basr" instruction instead, which does not have this limitation.
-m64-m31
When -m31 is specified, generate code compliant to the GNU/Linux
for S/390 ABI. When -m64 is specified, generate code compliant to
the GNU/Linux for zSeries ABI. This allows GCC in particular to
generate 64-bit instructions. For the s390 targets, the default is
-m31, while the s390x targets default to -m64.
-mzarch-mesa
When -mzarch is specified, generate code using the instructions
available on z/Architecture. When -mesa is specified, generate
code using the instructions available on ESA/390. Note that -mesa
is not possible with -m64. When generating code compliant to the
GNU/Linux for S/390 ABI, the default is -mesa. When generating
code compliant to the GNU/Linux for zSeries ABI, the default is
-mzarch.
-mmvcle-mno-mvcle
Generate (or do not generate) code using the "mvcle" instruction to
perform block moves. When -mno-mvcle is specified, use a "mvc"
loop instead. This is the default unless optimizing for size.
-mdebug-mno-debug
Print (or do not print) additional debug information when
compiling. The default is to not print debug information.
-march=cpu-type
Generate code that will run on cpu-type, which is the name of a
system representing a certain processor type. Possible values for
cpu-type are g5, g6, z900, z990, z9-109, z9-ec and z10. When
generating code using the instructions available on z/Architecture,
the default is -march=z900. Otherwise, the default is -march=g5.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code,
except for the ABI and the set of available instructions. The list
of cpu-type values is the same as for -march. The default is the
value used for -march.
-mtpf-trace-mno-tpf-trace
Generate code that adds (does not add) in TPF OS specific branches
to trace routines in the operating system. This option is off by
default, even when compiling for the TPF OS.
-mfused-madd-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply
and accumulate instructions. These instructions are generated by
default if hardware floating point is used.
-mwarn-framesize=framesize
Emit a warning if the current function exceeds the given frame
size. Because this is a compile-time check it doesn't need to be a
real problem when the program runs. It is intended to identify
functions that most probably cause a stack overflow. It is useful
to be used in an environment with limited stack size e.g. the linux
kernel.
-mwarn-dynamicstack
Emit a warning if the function calls alloca or uses dynamically
sized arrays. This is generally a bad idea with a limited stack
size.
-mstack-guard=stack-guard-mstack-size=stack-size
If these options are provided the s390 back end emits additional
instructions in the function prologue which trigger a trap if the
stack size is stack-guard bytes above the stack-size (remember that
the stack on s390 grows downward). If the stack-guard option is
omitted the smallest power of 2 larger than the frame size of the
compiled function is chosen. These options are intended to be used
to help debugging stack overflow problems. The additionally
emitted code causes only little overhead and hence can also be used
in production like systems without greater performance degradation.
The given values have to be exact powers of 2 and stack-size has to
be greater than stack-guard without exceeding 64k. In order to be
efficient the extra code makes the assumption that the stack starts
at an address aligned to the value given by stack-size. The stack-guard option can only be used in conjunction with stack-size.
ScoreOptions
These options are defined for Score implementations:
-meb
Compile code for big-endian mode. This is the default.
-mel
Compile code for little-endian mode.
-mnhwloop
Disable generate bcnz instruction.
-muls
Enable generate unaligned load and store instruction.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by
default.
-mscore5
Specify the SCORE5 as the target architecture.
-mscore5u
Specify the SCORE5U of the target architecture.
-mscore7
Specify the SCORE7 as the target architecture. This is the default.
-mscore7d
Specify the SCORE7D as the target architecture.
SHOptions
These -m options are defined for the SH implementations:
-m1 Generate code for the SH1.
-m2 Generate code for the SH2.
-m2e
Generate code for the SH2e.
-m2a-nofpu
Generate code for the SH2a without FPU, or for a SH2a-FPU in such a
way that the floating-point unit is not used.
-m2a-single-only
Generate code for the SH2a-FPU, in such a way that no double-
precision floating-point operations are used.
-m2a-single
Generate code for the SH2a-FPU assuming the floating-point unit is
in single-precision mode by default.
-m2a
Generate code for the SH2a-FPU assuming the floating-point unit is
in double-precision mode by default.
-m3 Generate code for the SH3.
-m3e
Generate code for the SH3e.
-m4-nofpu
Generate code for the SH4 without a floating-point unit.
-m4-single-only
Generate code for the SH4 with a floating-point unit that only
supports single-precision arithmetic.
-m4-single
Generate code for the SH4 assuming the floating-point unit is in
single-precision mode by default.
-m4 Generate code for the SH4.
-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a way that
the floating-point unit is not used.
-m4a-single-only
Generate code for the SH4a, in such a way that no double-precision
floating-point operations are used.
-m4a-single
Generate code for the SH4a assuming the floating-point unit is in
single-precision mode by default.
-m4a
Generate code for the SH4a.
-m4al
Same as -m4a-nofpu, except that it implicitly passes -dsp to the
assembler. GCC doesn't generate any DSP instructions at the
moment.
-mb Compile code for the processor in big-endian mode.
-ml Compile code for the processor in little-endian mode.
-mdalign
Align doubles at 64-bit boundaries. Note that this changes the
calling conventions, and thus some functions from the standard C
library will not work unless you recompile it first with -mdalign.
-mrelax
Shorten some address references at link time, when possible; uses
the linker option -relax.
-mbigtable
Use 32-bit offsets in "switch" tables. The default is to use
16-bit offsets.
-mbitops
Enable the use of bit manipulation instructions on SH2A.
-mfmovd
Enable the use of the instruction "fmovd". Check -mdalign for
alignment constraints.
-mhitachi
Comply with the calling conventions defined by Renesas.
-mrenesas
Comply with the calling conventions defined by Renesas.
-mno-renesas
Comply with the calling conventions defined for GCC before the
Renesas conventions were available. This option is the default for
all targets of the SH toolchain.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mhitachi is
given.
-mieee-mno-ieee
Control the IEEE compliance of floating-point comparisons, which
affects the handling of cases where the result of a comparison is
unordered. By default -mieee is implicitly enabled. If
-ffinite-math-only is enabled -mno-ieee is implicitly set, which
results in faster floating-point greater-equal and less-equal
comparisons. The implcit settings can be overridden by specifying
either -mieee or -mno-ieee.
-minline-ic_invalidate
Inline code to invalidate instruction cache entries after setting
up nested function trampolines. This option has no effect if
-musermode is in effect and the selected code generation option
(e.g. -m4) does not allow the use of the icbi instruction. If the
selected code generation option does not allow the use of the icbi
instruction, and -musermode is not in effect, the inlined code will
manipulate the instruction cache address array directly with an
associative write. This not only requires privileged mode, but it
will also fail if the cache line had been mapped via the TLB and
has become unmapped.
-misize
Dump instruction size and location in the assembly code.
-mpadstruct
This option is deprecated. It pads structures to multiple of 4
bytes, which is incompatible with the SH ABI.
-msoft-atomic
Generate GNU/Linux compatible gUSA software atomic sequences for
the atomic built-in functions. The generated atomic sequences
require support from the interrupt / exception handling code of the
system and are only suitable for single-core systems. They will
not perform correctly on multi-core systems. This option is
enabled by default when the target is "sh-*-linux*". For details
on the atomic built-in functions see __atomicBuiltins.
-mspace
Optimize for space instead of speed. Implied by -Os.
-mprefergot
When generating position-independent code, emit function calls
using the Global Offset Table instead of the Procedure Linkage
Table.
-musermode
Don't generate privileged mode only code; implies
-mno-inline-ic_invalidate if the inlined code would not work in
user mode. This is the default when the target is "sh-*-linux*".
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to be used for integer division
operations. For SHmedia strategy can be one of:
fp Performs the operation in floating point. This has a very high
latency, but needs only a few instructions, so it might be a
good choice if your code has enough easily-exploitable ILP to
allow the compiler to schedule the floating-point instructions
together with other instructions. Division by zero causes a
floating-point exception.
inv Uses integer operations to calculate the inverse of the
divisor, and then multiplies the dividend with the inverse.
This strategy allows CSE and hoisting of the inverse
calculation. Division by zero calculates an unspecified
result, but does not trap.
inv:minlat
A variant of inv where, if no CSE or hoisting opportunities
have been found, or if the entire operation has been hoisted to
the same place, the last stages of the inverse calculation are
intertwined with the final multiply to reduce the overall
latency, at the expense of using a few more instructions, and
thus offering fewer scheduling opportunities with other code.
call
Calls a library function that usually implements the inv:minlat
strategy. This gives high code density for "m5-*media-nofpu"
compilations.
call2
Uses a different entry point of the same library function,
where it assumes that a pointer to a lookup table has already
been set up, which exposes the pointer load to CSE and code
hoisting optimizations.
inv:callinv:call2inv:fp
Use the inv algorithm for initial code generation, but if the
code stays unoptimized, revert to the call, call2, or fp
strategies, respectively. Note that the potentially-trapping
side effect of division by zero is carried by a separate
instruction, so it is possible that all the integer
instructions are hoisted out, but the marker for the side
effect stays where it is. A recombination to floating-point
operations or a call is not possible in that case.
inv20uinv20l
Variants of the inv:minlat strategy. In the case that the
inverse calculation is not separated from the multiply, they
speed up division where the dividend fits into 20 bits (plus
sign where applicable) by inserting a test to skip a number of
operations in this case; this test slows down the case of
larger dividends. inv20u assumes the case of a such a small
dividend to be unlikely, and inv20l assumes it to be likely.
For targets other than SHmedia strategy can be one of:
call-div1
Calls a library function that uses the single-step division
instruction "div1" to perform the operation. Division by zero
calculates an unspecified result and does not trap. This is
the default except for SH4, SH2A and SHcompact.
call-fp
Calls a library function that performs the operation in double
precision floating point. Division by zero causes a floating-
point exception. This is the default for SHcompact with FPU.
Specifying this for targets that do not have a double precision
FPU will default to "call-div1".
call-table
Calls a library function that uses a lookup table for small
divisors and the "div1" instruction with case distinction for
larger divisors. Division by zero calculates an unspecified
result and does not trap. This is the default for SH4.
Specifying this for targets that do not have dynamic shift
instructions will default to "call-div1".
When a division strategy has not been specified the default
strategy will be selected based on the current target. For SH2A
the default strategy is to use the "divs" and "divu" instructions
instead of library function calls.
-maccumulate-outgoing-args
Reserve space once for outgoing arguments in the function prologue
rather than around each call. Generally beneficial for performance
and size. Also needed for unwinding to avoid changing the stack
frame around conditional code.
-mdivsi3_libfunc=name
Set the name of the library function used for 32-bit signed
division to name. This only affect the name used in the call and
inv:call division strategies, and the compiler will still expect
the same sets of input/output/clobbered registers as if this option
was not present.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers.
A fixed register is one that the register allocator can not use.
This is useful when compiling kernel code. A register range is
specified as two registers separated by a dash. Multiple register
ranges can be specified separated by a comma.
-madjust-unroll
Throttle unrolling to avoid thrashing target registers. This
option only has an effect if the gcc code base supports the
TARGET_ADJUST_UNROLL_MAX target hook.
-mindexed-addressing
Enable the use of the indexed addressing mode for
SHmedia32/SHcompact. This is only safe if the hardware and/or OS
implement 32-bit wrap-around semantics for the indexed addressing
mode. The architecture allows the implementation of processors
with 64-bit MMU, which the OS could use to get 32-bit addressing,
but since no current hardware implementation supports this or any
other way to make the indexed addressing mode safe to use in the
32-bit ABI, the default is -mno-indexed-addressing.
-mgettrcost=number
Set the cost assumed for the gettr instruction to number. The
default is 2 if -mpt-fixed is in effect, 100 otherwise.
-mpt-fixed
Assume pt* instructions won't trap. This will generally generate
better scheduled code, but is unsafe on current hardware. The
current architecture definition says that ptabs and ptrel trap when
the target anded with 3 is 3. This has the unintentional effect of
making it unsafe to schedule ptabs / ptrel before a branch, or
hoist it out of a loop. For example, __do_global_ctors, a part of
libgcc that runs constructors at program startup, calls functions
in a list which is delimited by -1. With the -mpt-fixed option,
the ptabs will be done before testing against -1. That means that
all the constructors will be run a bit quicker, but when the loop
comes to the end of the list, the program crashes because ptabs
loads -1 into a target register. Since this option is unsafe for
any hardware implementing the current architecture specification,
the default is -mno-pt-fixed. Unless the user specifies a specific
cost with -mgettrcost, -mno-pt-fixed also implies -mgettrcost=100;
this deters register allocation using target registers for storing
ordinary integers.
-minvalid-symbols
Assume symbols might be invalid. Ordinary function symbols
generated by the compiler will always be valid to load with
movi/shori/ptabs or movi/shori/ptrel, but with assembler and/or
linker tricks it is possible to generate symbols that will cause
ptabs / ptrel to trap. This option is only meaningful when
-mno-pt-fixed is in effect. It will then prevent cross-basic-block
cse, hoisting and most scheduling of symbol loads. The default is
-mno-invalid-symbols.
-mbranch-cost=num
Assume num to be the cost for a branch instruction. Higher numbers
will make the compiler try to generate more branch-free code if
possible. If not specified the value is selected depending on the
processor type that is being compiled for.
-mcbranchdi
Enable the "cbranchdi4" instruction pattern.
-mcmpeqdi
Emit the "cmpeqdi_t" instruction pattern even when -mcbranchdi is
in effect.
-mfused-madd
Allow the usage of the "fmac" instruction (floating-point multiply-
accumulate) if the processor type supports it. Enabling this
option might generate code that produces different numeric
floating-point results compared to strict IEEE 754 arithmetic.
-mpretend-cmove
Prefer zero-displacement conditional branches for conditional move
instruction patterns. This can result in faster code on the SH4
processor.
Solaris2Options
These -m options are supported on Solaris 2:
-mimpure-text-mimpure-text, used in addition to -shared, tells the compiler to
not pass -ztext to the linker when linking a shared object. Using
this option, you can link position-dependent code into a shared
object.
-mimpure-text suppresses the "relocations remain against
allocatable but non-writable sections" linker error message.
However, the necessary relocations will trigger copy-on-write, and
the shared object is not actually shared across processes. Instead
of using -mimpure-text, you should compile all source code with
-fpic or -fPIC.
These switches are supported in addition to the above on Solaris 2:
-pthreads
Add support for multithreading using the POSIX threads library.
This option sets flags for both the preprocessor and linker. This
option does not affect the thread safety of object code produced
by the compiler or that of libraries supplied with it.
-pthread
This is a synonym for -pthreads.
SPARCOptions
These -m options are supported on the SPARC:
-mno-app-regs-mapp-regs
Specify -mapp-regs to generate output using the global registers 2
through 4, which the SPARC SVR4 ABI reserves for applications.
Like the global register 1, each global register 2 through 4 is
then treated as an allocable register that is clobbered by function
calls. This is the default.
To be fully SVR4 ABI compliant at the cost of some performance
loss, specify -mno-app-regs. You should compile libraries and
system software with this option.
-mflat-mno-flat
With -mflat, the compiler does not generate save/restore
instructions and uses a "flat" or single register window model.
This model is compatible with the regular register window model.
The local registers and the input registers (0--5) are still
treated as "call-saved" registers and will be saved on the stack as
needed.
With -mno-flat (the default), the compiler generates save/restore
instructions (except for leaf functions). This is the normal
operating mode.
-mfpu-mhard-float
Generate output containing floating-point instructions. This is
the default.
-mno-fpu-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all SPARC
targets. Normally the facilities of the machine's usual C compiler
are used, but this cannot be done directly in cross-compilation.
You must make your own arrangements to provide suitable library
functions for cross-compilation. The embedded targets sparc-*-aout
and sparclite-*-* do provide software floating-point support.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this to
work.
-mhard-quad-float
Generate output containing quad-word (long double) floating-point
instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long
double) floating-point instructions. The functions called are
those specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have
hardware support for the quad-word floating-point instructions.
They all invoke a trap handler for one of these instructions, and
then the trap handler emulates the effect of the instruction.
Because of the trap handler overhead, this is much slower than
calling the ABI library routines. Thus the -msoft-quad-float
option is the default.
-mno-unaligned-doubles-munaligned-doubles
Assume that doubles have 8-byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8-byte
alignment only if they are contained in another type, or if they
have an absolute address. Otherwise, it assumes they have 4-byte
alignment. Specifying this option avoids some rare compatibility
problems with code generated by other compilers. It is not the
default because it results in a performance loss, especially for
floating-point code.
-mno-faster-structs-mfaster-structs
With -mfaster-structs, the compiler assumes that structures should
have 8-byte alignment. This enables the use of pairs of "ldd" and
"std" instructions for copies in structure assignment, in place of
twice as many "ld" and "st" pairs. However, the use of this
changed alignment directly violates the SPARC ABI. Thus, it's
intended only for use on targets where the developer acknowledges
that their resulting code will not be directly in line with the
rules of the ABI.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling
parameters for machine type cpu_type. Supported values for
cpu_type are v7, cypress, v8, supersparc, hypersparc, leon,
sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9,
ultrasparc, ultrasparc3, niagara, niagara2, niagara3, and niagara4.
Native Solaris and GNU/Linux toolchains also support the value
native, which selects the best architecture option for the host
processor. -mcpu=native has no effect if GCC does not recognize
the processor.
Default instruction scheduling parameters are used for values that
select an architecture and not an implementation. These are v7,
v8, sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported
implementations.
v7 cypress
v8 supersparc, hypersparc, leon
sparclite
f930, f934, sparclite86x
sparclet
tsc701
v9 ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4
By default (unless configured otherwise), GCC generates code for
the V7 variant of the SPARC architecture. With -mcpu=cypress, the
compiler additionally optimizes it for the Cypress CY7C602 chip, as
used in the SPARCStation/SPARCServer 3xx series. This is also
appropriate for the older SPARCStation 1, 2, IPX etc.
With -mcpu=v8, GCC generates code for the V8 variant of the SPARC
architecture. The only difference from V7 code is that the
compiler emits the integer multiply and integer divide instructions
which exist in SPARC-V8 but not in SPARC-V7. With
-mcpu=supersparc, the compiler additionally optimizes it for the
SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000
series.
With -mcpu=sparclite, GCC generates code for the SPARClite variant
of the SPARC architecture. This adds the integer multiply, integer
divide step and scan ("ffs") instructions which exist in SPARClite
but not in SPARC-V7. With -mcpu=f930, the compiler additionally
optimizes it for the Fujitsu MB86930 chip, which is the original
SPARClite, with no FPU. With -mcpu=f934, the compiler additionally
optimizes it for the Fujitsu MB86934 chip, which is the more recent
SPARClite with FPU.
With -mcpu=sparclet, GCC generates code for the SPARClet variant of
the SPARC architecture. This adds the integer multiply,
multiply/accumulate, integer divide step and scan ("ffs")
instructions which exist in SPARClet but not in SPARC-V7. With
-mcpu=tsc701, the compiler additionally optimizes it for the TEMIC
SPARClet chip.
With -mcpu=v9, GCC generates code for the V9 variant of the SPARC
architecture. This adds 64-bit integer and floating-point move
instructions, 3 additional floating-point condition code registers
and conditional move instructions. With -mcpu=ultrasparc, the
compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi
chips. With -mcpu=ultrasparc3, the compiler additionally optimizes
it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With
-mcpu=niagara, the compiler additionally optimizes it for Sun
UltraSPARC T1 chips. With -mcpu=niagara2, the compiler
additionally optimizes it for Sun UltraSPARC T2 chips. With
-mcpu=niagara3, the compiler additionally optimizes it for Sun
UltraSPARC T3 chips. With -mcpu=niagara4, the compiler
additionally optimizes it for Sun UltraSPARC T4 chips.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the instruction set or register set that
the option -mcpu=cpu_type would.
The same values for -mcpu=cpu_type can be used for -mtune=cpu_type,
but the only useful values are those that select a particular CPU
implementation. Those are cypress, supersparc, hypersparc, leon,
f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara,
niagara2, niagara3 and niagara4. With native Solaris and GNU/Linux
toolchains, native can also be used.
-mv8plus-mno-v8plus
With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The
difference from the V8 ABI is that the global and out registers are
considered 64 bits wide. This is enabled by default on Solaris in
32-bit mode for all SPARC-V9 processors.
-mvis-mno-vis
With -mvis, GCC generates code that takes advantage of the
UltraSPARC Visual Instruction Set extensions. The default is
-mno-vis.
-mvis2-mno-vis2
With -mvis2, GCC generates code that takes advantage of version 2.0
of the UltraSPARC Visual Instruction Set extensions. The default
is -mvis2 when targetting a cpu that supports such instructions,
such as UltraSPARC-III and later. Setting -mvis2 also sets -mvis.
-mvis3-mno-vis3
With -mvis3, GCC generates code that takes advantage of version 3.0
of the UltraSPARC Visual Instruction Set extensions. The default
is -mvis3 when targetting a cpu that supports such instructions,
such as niagara-3 and later. Setting -mvis3 also sets -mvis2 and
-mvis.
-mpopc-mno-popc
With -mpopc, GCC generates code that takes advantage of the
UltraSPARC population count instruction. The default is -mpopc
when targetting a cpu that supports such instructions, such as
Niagara-2 and later.
-mfmaf-mno-fmaf
With -mfmaf, GCC generates code that takes advantage of the
UltraSPARC Fused Multiply-Add Floating-point extensions. The
default is -mfmaf when targetting a cpu that supports such
instructions, such as Niagara-3 and later.
-mfix-at697f
Enable the documented workaround for the single erratum of the
Atmel AT697F processor (which corresponds to erratum #13 of the
AT697E processor).
These -m options are supported in addition to the above on SPARC-V9
processors in 64-bit environments:
-m32-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit
environment sets int, long and pointer to 32 bits. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits.
-mcmodel=which
Set the code model to one of
medlow
The Medium/Low code model: 64-bit addresses, programs must be
linked in the low 32 bits of memory. Programs can be
statically or dynamically linked.
medmid
The Medium/Middle code model: 64-bit addresses, programs must
be linked in the low 44 bits of memory, the text and data
segments must be less than 2GB in size and the data segment
must be located within 2GB of the text segment.
medany
The Medium/Anywhere code model: 64-bit addresses, programs may
be linked anywhere in memory, the text and data segments must
be less than 2GB in size and the data segment must be located
within 2GB of the text segment.
embmedany
The Medium/Anywhere code model for embedded systems: 64-bit
addresses, the text and data segments must be less than 2GB in
size, both starting anywhere in memory (determined at link
time). The global register %g4 points to the base of the data
segment. Programs are statically linked and PIC is not
supported.
-mmemory-model=mem-model
Set the memory model in force on the processor to one of
default
The default memory model for the processor and operating
system.
rmo Relaxed Memory Order
pso Partial Store Order
tso Total Store Order
sc Sequential Consistency
These memory models are formally defined in Appendix D of the Sparc
V9 architecture manual, as set in the processor's "PSTATE.MM"
field.
-mstack-bias-mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer, and frame
pointer if present, are offset by -2047 which must be added back
when making stack frame references. This is the default in 64-bit
mode. Otherwise, assume no such offset is present.
SPUOptions
These -m options are supported on the SPU:
-mwarn-reloc-merror-reloc
The loader for SPU does not handle dynamic relocations. By
default, GCC will give an error when it generates code that
requires a dynamic relocation. -mno-error-reloc disables the
error, -mwarn-reloc will generate a warning instead.
-msafe-dma-munsafe-dma
Instructions that initiate or test completion of DMA must not be
reordered with respect to loads and stores of the memory that is
being accessed. Users typically address this problem using the
volatile keyword, but that can lead to inefficient code in places
where the memory is known to not change. Rather than mark the
memory as volatile we treat the DMA instructions as potentially
effecting all memory. With -munsafe-dma users must use the
volatile keyword to protect memory accesses.
-mbranch-hints
By default, GCC will generate a branch hint instruction to avoid
pipeline stalls for always taken or probably taken branches. A
hint will not be generated closer than 8 instructions away from its
branch. There is little reason to disable them, except for
debugging purposes, or to make an object a little bit smaller.
-msmall-mem-mlarge-mem
By default, GCC generates code assuming that addresses are never
larger than 18 bits. With -mlarge-mem code is generated that
assumes a full 32-bit address.
-mstdmain
By default, GCC links against startup code that assumes the SPU-
style main function interface (which has an unconventional
parameter list). With -mstdmain, GCC will link your program
against startup code that assumes a C99-style interface to "main",
including a local copy of "argv" strings.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers.
A fixed register is one that the register allocator can not use.
This is useful when compiling kernel code. A register range is
specified as two registers separated by a dash. Multiple register
ranges can be specified separated by a comma.
-mea32-mea64
Compile code assuming that pointers to the PPU address space
accessed via the "__ea" named address space qualifier are either 32
or 64 bits wide. The default is 32 bits. As this is an ABI
changing option, all object code in an executable must be compiled
with the same setting.
-maddress-space-conversion-mno-address-space-conversion
Allow/disallow treating the "__ea" address space as superset of the
generic address space. This enables explicit type casts between
"__ea" and generic pointer as well as implicit conversions of
generic pointers to "__ea" pointers. The default is to allow
address space pointer conversions.
-mcache-size=cache-size
This option controls the version of libgcc that the compiler links
to an executable and selects a software-managed cache for accessing
variables in the "__ea" address space with a particular cache size.
Possible options for cache-size are 8, 16, 32, 64 and 128. The
default cache size is 64KB.
-matomic-updates-mno-atomic-updates
This option controls the version of libgcc that the compiler links
to an executable and selects whether atomic updates to the
software-managed cache of PPU-side variables are used. If you use
atomic updates, changes to a PPU variable from SPU code using the
"__ea" named address space qualifier will not interfere with
changes to other PPU variables residing in the same cache line from
PPU code. If you do not use atomic updates, such interference may
occur; however, writing back cache lines will be more efficient.
The default behavior is to use atomic updates.
-mdual-nops-mdual-nops=n
By default, GCC will insert nops to increase dual issue when it
expects it to increase performance. n can be a value from 0 to 10.
A smaller n will insert fewer nops. 10 is the default, 0 is the
same as -mno-dual-nops. Disabled with -Os.
-mhint-max-nops=n
Maximum number of nops to insert for a branch hint. A branch hint
must be at least 8 instructions away from the branch it is
effecting. GCC will insert up to n nops to enforce this, otherwise
it will not generate the branch hint.
-mhint-max-distance=n
The encoding of the branch hint instruction limits the hint to be
within 256 instructions of the branch it is effecting. By default,
GCC makes sure it is within 125.
-msafe-hints
Work around a hardware bug that causes the SPU to stall
indefinitely. By default, GCC will insert the "hbrp" instruction
to make sure this stall won't happen.
OptionsforSystemV
These additional options are available on System V Release 4 for
compatibility with other compilers on those systems:
-G Create a shared object. It is recommended that -symbolic or
-shared be used instead.
-Qy Identify the versions of each tool used by the compiler, in a
".ident" assembler directive in the output.
-Qn Refrain from adding ".ident" directives to the output file (this is
the default).
-YP,dirs
Search the directories dirs, and no others, for libraries specified
with -l.
-Ym,dir
Look in the directory dir to find the M4 preprocessor. The
assembler uses this option.
TILE-GxOptions
These -m options are supported on the TILE-Gx:
-mcpu=name
Selects the type of CPU to be targeted. Currently the only
supported type is tilegx.
-m32-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit
environment sets int, long, and pointer to 32 bits. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits.
TILEProOptions
These -m options are supported on the TILEPro:
-mcpu=name
Selects the type of CPU to be targeted. Currently the only
supported type is tilepro.
-m32
Generate code for a 32-bit environment, which sets int, long, and
pointer to 32 bits. This is the only supported behavior so the
flag is essentially ignored.
V850Options
These -m options are defined for V850 implementations:
-mlong-calls-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to
be far away, the compiler will always load the functions address up
into a register, and call indirect through the pointer.
-mno-ep-mep
Do not optimize (do optimize) basic blocks that use the same index
pointer 4 or more times to copy pointer into the "ep" register, and
use the shorter "sld" and "sst" instructions. The -mep option is
on by default if you optimize.
-mno-prolog-function-mprolog-function
Do not use (do use) external functions to save and restore
registers at the prologue and epilogue of a function. The external
functions are slower, but use less code space if more than one
function saves the same number of registers. The -mprolog-function
option is on by default if you optimize.
-mspace
Try to make the code as small as possible. At present, this just
turns on the -mep and -mprolog-function options.
-mtda=n
Put static or global variables whose size is n bytes or less into
the tiny data area that register "ep" points to. The tiny data
area can hold up to 256 bytes in total (128 bytes for byte
references).
-msda=n
Put static or global variables whose size is n bytes or less into
the small data area that register "gp" points to. The small data
area can hold up to 64 kilobytes.
-mzda=n
Put static or global variables whose size is n bytes or less into
the first 32 kilobytes of memory.
-mv850
Specify that the target processor is the V850.
-mbig-switch
Generate code suitable for big switch tables. Use this option only
if the assembler/linker complain about out of range branches within
a switch table.
-mapp-regs
This option will cause r2 and r5 to be used in the code generated
by the compiler. This setting is the default.
-mno-app-regs
This option will cause r2 and r5 to be treated as fixed registers.
-mv850e2v3
Specify that the target processor is the V850E2V3. The
preprocessor constants __v850e2v3__ will be defined if this option
is used.
-mv850e2
Specify that the target processor is the V850E2. The preprocessor
constants __v850e2__ will be defined if this option is used.
-mv850e1
Specify that the target processor is the V850E1. The preprocessor
constants __v850e1__ and __v850e__ will be defined if this option
is used.
-mv850es
Specify that the target processor is the V850ES. This is an alias
for the -mv850e1 option.
-mv850e
Specify that the target processor is the V850E. The preprocessor
constant __v850e__ will be defined if this option is used.
If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor
-mv850e2v3 are defined then a default target processor will be
chosen and the relevant __v850*__ preprocessor constant will be
defined.
The preprocessor constants __v850 and __v851__ are always defined,
regardless of which processor variant is the target.
-mdisable-callt
This option will suppress generation of the CALLT instruction for
the v850e, v850e1, v850e2 and v850e2v3 flavors of the v850
architecture. The default is -mno-disable-callt which allows the
CALLT instruction to be used.
VAXOptions
These -m options are defined for the VAX:
-munix
Do not output certain jump instructions ("aobleq" and so on) that
the Unix assembler for the VAX cannot handle across long ranges.
-mgnu
Do output those jump instructions, on the assumption that you will
assemble with the GNU assembler.
-mg Output code for G-format floating-point numbers instead of
D-format.
VxWorksOptions
The options in this section are defined for all VxWorks targets.
Options specific to the target hardware are listed with the other
options for that target.
-mrtp
GCC can generate code for both VxWorks kernels and real time
processes (RTPs). This option switches from the former to the
latter. It also defines the preprocessor macro "__RTP__".
-non-static
Link an RTP executable against shared libraries rather than static
libraries. The options -static and -shared can also be used for
RTPs; -static is the default.
-Bstatic-Bdynamic
These options are passed down to the linker. They are defined for
compatibility with Diab.
-Xbind-lazy
Enable lazy binding of function calls. This option is equivalent
to -Wl,-z,now and is defined for compatibility with Diab.
-Xbind-now
Disable lazy binding of function calls. This option is the default
and is defined for compatibility with Diab.
x86-64Options
These are listed under
Xstormy16Options
These options are defined for Xstormy16:
-msim
Choose startup files and linker script suitable for the simulator.
XtensaOptions
These options are supported for Xtensa targets:
-mconst16-mno-const16
Enable or disable use of "CONST16" instructions for loading
constant values. The "CONST16" instruction is currently not a
standard option from Tensilica. When enabled, "CONST16"
instructions are always used in place of the standard "L32R"
instructions. The use of "CONST16" is enabled by default only if
the "L32R" instruction is not available.
-mfused-madd-mno-fused-madd
Enable or disable use of fused multiply/add and multiply/subtract
instructions in the floating-point option. This has no effect if
the floating-point option is not also enabled. Disabling fused
multiply/add and multiply/subtract instructions forces the compiler
to use separate instructions for the multiply and add/subtract
operations. This may be desirable in some cases where strict IEEE
754-compliant results are required: the fused multiply add/subtract
instructions do not round the intermediate result, thereby
producing results with more bits of precision than specified by the
IEEE standard. Disabling fused multiply add/subtract instructions
also ensures that the program output is not sensitive to the
compiler's ability to combine multiply and add/subtract operations.
-mserialize-volatile-mno-serialize-volatile
When this option is enabled, GCC inserts "MEMW" instructions before
"volatile" memory references to guarantee sequential consistency.
The default is -mserialize-volatile. Use -mno-serialize-volatile
to omit the "MEMW" instructions.
-mforce-no-pic
For targets, like GNU/Linux, where all user-mode Xtensa code must
be position-independent code (PIC), this option disables PIC for
compiling kernel code.
-mtext-section-literals-mno-text-section-literals
Control the treatment of literal pools. The default is
-mno-text-section-literals, which places literals in a separate
section in the output file. This allows the literal pool to be
placed in a data RAM/ROM, and it also allows the linker to combine
literal pools from separate object files to remove redundant
literals and improve code size. With -mtext-section-literals, the
literals are interspersed in the text section in order to keep them
as close as possible to their references. This may be necessary
for large assembly files.
-mtarget-align-mno-target-align
When this option is enabled, GCC instructs the assembler to
automatically align instructions to reduce branch penalties at the
expense of some code density. The assembler attempts to widen
density instructions to align branch targets and the instructions
following call instructions. If there are not enough preceding
safe density instructions to align a target, no widening will be
performed. The default is -mtarget-align. These options do not
affect the treatment of auto-aligned instructions like "LOOP",
which the assembler will always align, either by widening density
instructions or by inserting no-op instructions.
-mlongcalls-mno-longcalls
When this option is enabled, GCC instructs the assembler to
translate direct calls to indirect calls unless it can determine
that the target of a direct call is in the range allowed by the
call instruction. This translation typically occurs for calls to
functions in other source files. Specifically, the assembler
translates a direct "CALL" instruction into an "L32R" followed by a
"CALLX" instruction. The default is -mno-longcalls. This option
should be used in programs where the call target can potentially be
out of range. This option is implemented in the assembler, not the
compiler, so the assembly code generated by GCC will still show
direct call instructions---look at the disassembled object code to
see the actual instructions. Note that the assembler will use an
indirect call for every cross-file call, not just those that really
will be out of range.
zSeriesOptions
These are listed under
OptionsforCodeGenerationConventions
These machine-independent options control the interface conventions
used in code generation.
Most of them have both positive and negative forms; the negative form
of -ffoo would be -fno-foo. In the table below, only one of the forms
is listed---the one that is not the default. You can figure out the
other form by either removing no- or adding it.
-fbounds-check
For front ends that support it, generate additional code to check
that indices used to access arrays are within the declared range.
This is currently only supported by the Java and Fortran front
ends, where this option defaults to true and false respectively.
-ftrapv
This option generates traps for signed overflow on addition,
subtraction, multiplication operations.
-fwrapv
This option instructs the compiler to assume that signed arithmetic
overflow of addition, subtraction and multiplication wraps around
using twos-complement representation. This flag enables some
optimizations and disables others. This option is enabled by
default for the Java front end, as required by the Java language
specification.
-fexceptions
Enable exception handling. Generates extra code needed to
propagate exceptions. For some targets, this implies GCC will
generate frame unwind information for all functions, which can
produce significant data size overhead, although it does not affect
execution. If you do not specify this option, GCC will enable it
by default for languages like C++ that normally require exception
handling, and disable it for languages like C that do not normally
require it. However, you may need to enable this option when
compiling C code that needs to interoperate properly with exception
handlers written in C++. You may also wish to disable this option
if you are compiling older C++ programs that don't use exception
handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw
exceptions. Note that this requires platform-specific runtime
support that does not exist everywhere. Moreover, it only allows
trapping instructions to throw exceptions, i.e. memory references
or floating-point instructions. It does not allow exceptions to be
thrown from arbitrary signal handlers such as "SIGALRM".
-funwind-tables
Similar to -fexceptions, except that it will just generate any
needed static data, but will not affect the generated code in any
other way. You will normally not enable this option; instead, a
language processor that needs this handling would enable it on your
behalf.
-fasynchronous-unwind-tables
Generate unwind table in dwarf2 format, if supported by target
machine. The table is exact at each instruction boundary, so it
can be used for stack unwinding from asynchronous events (such as
debugger or garbage collector).
-fpcc-struct-return
Return "short" "struct" and "union" values in memory like longer
ones, rather than in registers. This convention is less efficient,
but it has the advantage of allowing intercallability between GCC-
compiled files and files compiled with other compilers,
particularly the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends
on the target configuration macros.
Short structures and unions are those whose size and alignment
match that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not
binary compatible with code compiled with the -freg-struct-return
switch. Use it to conform to a non-default application binary
interface.
-freg-struct-return
Return "struct" and "union" values in registers when possible.
This is more efficient for small structures than
-fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return,
GCC defaults to whichever convention is standard for the target.
If there is no standard convention, GCC defaults to
-fpcc-struct-return, except on targets where GCC is the principal
compiler. In those cases, we can choose the standard, and we chose
the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return switch is not
binary compatible with code compiled with the -fpcc-struct-return
switch. Use it to conform to a non-default application binary
interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the
declared range of possible values. Specifically, the "enum" type
will be equivalent to the smallest integer type that has enough
room.
Warning: the -fshort-enums switch causes GCC to generate code that
is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
-fshort-double
Use the same size for "double" as for "float".
Warning: the -fshort-double switch causes GCC to generate code that
is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
-fshort-wchar
Override the underlying type for wchar_t to be shortunsignedint
instead of the default for the target. This option is useful for
building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code that
is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
-fno-common
In C code, controls the placement of uninitialized global
variables. Unix C compilers have traditionally permitted multiple
definitions of such variables in different compilation units by
placing the variables in a common block. This is the behavior
specified by -fcommon, and is the default for GCC on most targets.
On the other hand, this behavior is not required by ISO C, and on
some targets may carry a speed or code size penalty on variable
references. The -fno-common option specifies that the compiler
should place uninitialized global variables in the data section of
the object file, rather than generating them as common blocks.
This has the effect that if the same variable is declared (without
"extern") in two different compilations, you will get a multiple-
definition error when you link them. In this case, you must
compile with -fcommon instead. Compiling with -fno-common is
useful on targets for which it provides better performance, or if
you wish to verify that the program will work on other systems that
always treat uninitialized variable declarations this way.
-fno-ident
Ignore the #ident directive.
-finhibit-size-directive
Don't output a ".size" assembler directive, or anything else that
would cause trouble if the function is split in the middle, and the
two halves are placed at locations far apart in memory. This
option is used when compiling crtstuff.c; you should not need to
use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code to
make it more readable. This option is generally only of use to
those who actually need to read the generated assembly code
(perhaps while debugging the compiler itself).
-fno-verbose-asm, the default, causes the extra information to be
omitted and is useful when comparing two assembler files.
-frecord-gcc-switches
This switch causes the command line that was used to invoke the
compiler to be recorded into the object file that is being created.
This switch is only implemented on some targets and the exact
format of the recording is target and binary file format dependent,
but it usually takes the form of a section containing ASCII text.
This switch is related to the -fverbose-asm switch, but that switch
only records information in the assembler output file as comments,
so it never reaches the object file. See also
-grecord-gcc-switches for another way of storing compiler options
into the object file.
-fpic
Generate position-independent code (PIC) suitable for use in a
shared library, if supported for the target machine. Such code
accesses all constant addresses through a global offset table
(GOT). The dynamic loader resolves the GOT entries when the
program starts (the dynamic loader is not part of GCC; it is part
of the operating system). If the GOT size for the linked
executable exceeds a machine-specific maximum size, you get an
error message from the linker indicating that -fpic does not work;
in that case, recompile with -fPIC instead. (These maximums are 8k
on the SPARC and 32k on the m68k and RS/6000. The 386 has no such
limit.)
Position-independent code requires special support, and therefore
works only on certain machines. For the 386, GCC supports PIC for
System V but not for the Sun 386i. Code generated for the IBM
RS/6000 is always position-independent.
When this flag is set, the macros "__pic__" and "__PIC__" are
defined to 1.
-fPIC
If supported for the target machine, emit position-independent
code, suitable for dynamic linking and avoiding any limit on the
size of the global offset table. This option makes a difference on
the m68k, PowerPC and SPARC.
Position-independent code requires special support, and therefore
works only on certain machines.
When this flag is set, the macros "__pic__" and "__PIC__" are
defined to 2.
-fpie-fPIE
These options are similar to -fpic and -fPIC, but generated
position independent code can be only linked into executables.
Usually these options are used when -pie GCC option will be used
during linking.
-fpie and -fPIE both define the macros "__pie__" and "__PIE__".
The macros have the value 1 for -fpie and 2 for -fPIE.
-fno-jump-tables
Do not use jump tables for switch statements even where it would be
more efficient than other code generation strategies. This option
is of use in conjunction with -fpic or -fPIC for building code that
forms part of a dynamic linker and cannot reference the address of
a jump table. On some targets, jump tables do not require a GOT
and this option is not needed.
-ffixed-reg
Treat the register named reg as a fixed register; generated code
should never refer to it (except perhaps as a stack pointer, frame
pointer or in some other fixed role).
reg must be the name of a register. The register names accepted
are machine-specific and are defined in the "REGISTER_NAMES" macro
in the machine description macro file.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is
clobbered by function calls. It may be allocated for temporaries
or variables that do not live across a call. Functions compiled
this way will not save and restore the register reg.
It is an error to used this flag with the frame pointer or stack
pointer. Use of this flag for other registers that have fixed
pervasive roles in the machine's execution model will produce
disastrous results.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by
functions. It may be allocated even for temporaries or variables
that live across a call. Functions compiled this way will save and
restore the register reg if they use it.
It is an error to used this flag with the frame pointer or stack
pointer. Use of this flag for other registers that have fixed
pervasive roles in the machine's execution model will produce
disastrous results.
A different sort of disaster will result from the use of this flag
for a register in which function values may be returned.
This flag does not have a negative form, because it specifies a
three-way choice.
-fpack-struct[=n]
Without a value specified, pack all structure members together
without holes. When a value is specified (which must be a small
power of two), pack structure members according to this value,
representing the maximum alignment (that is, objects with default
alignment requirements larger than this will be output potentially
unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that
is not binary compatible with code generated without that switch.
Additionally, it makes the code suboptimal. Use it to conform to a
non-default application binary interface.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions.
Just after function entry and just before function exit, the
following profiling functions will be called with the address of
the current function and its call site. (On some platforms,
"__builtin_return_address" does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current
function, which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline in
other functions. The profiling calls will indicate where,
conceptually, the inline function is entered and exited. This
means that addressable versions of such functions must be
available. If all your uses of a function are expanded inline,
this may mean an additional expansion of code size. If you use
externinline in your C code, an addressable version of such
functions must be provided. (This is normally the case anyways,
but if you get lucky and the optimizer always expands the functions
inline, you might have gotten away without providing static
copies.)
A function may be given the attribute "no_instrument_function", in
which case this instrumentation will not be done. This can be
used, for example, for the profiling functions listed above, high-
priority interrupt routines, and any functions from which the
profiling functions cannot safely be called (perhaps signal
handlers, if the profiling routines generate output or allocate
memory).
-finstrument-functions-exclude-file-list=file,file,...
Set the list of functions that are excluded from instrumentation
(see the description of "-finstrument-functions"). If the file
that contains a function definition matches with one of file, then
that function is not instrumented. The match is done on
substrings: if the file parameter is a substring of the file name,
it is considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
will exclude any inline function defined in files whose pathnames
contain "/bits/stl" or "include/sys".
If, for some reason, you want to include letter ',' in one of sym,
write ','. For example,
"-finstrument-functions-exclude-file-list=',,tmp'" (note the single
quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...
This is similar to "-finstrument-functions-exclude-file-list", but
this option sets the list of function names to be excluded from
instrumentation. The function name to be matched is its user-
visible name, such as "vector<int> blah(const vector<int> &)", not
the internal mangled name (e.g., "_Z4blahRSt6vectorIiSaIiEE"). The
match is done on substrings: if the sym parameter is a substring of
the function name, it is considered to be a match. For C99 and C++
extended identifiers, the function name must be given in UTF-8, not
using universal character names.
-fstack-check
Generate code to verify that you do not go beyond the boundary of
the stack. You should specify this flag if you are running in an
environment with multiple threads, but only rarely need to specify
it in a single-threaded environment since stack overflow is
automatically detected on nearly all systems if there is only one
stack.
Note that this switch does not actually cause checking to be done;
the operating system or the language runtime must do that. The
switch causes generation of code to ensure that they see the stack
being extended.
You can additionally specify a string parameter: "no" means no
checking, "generic" means force the use of old-style checking,
"specific" means use the best checking method and is equivalent to
bare -fstack-check.
Old-style checking is a generic mechanism that requires no specific
target support in the compiler but comes with the following
drawbacks:
1. Modified allocation strategy for large objects: they will
always be allocated dynamically if their size exceeds a fixed
threshold.
2. Fixed limit on the size of the static frame of functions: when
it is topped by a particular function, stack checking is not
reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy
and the generic implementation, the performances of the code
are hampered.
Note that old-style stack checking is also the fallback method for
"specific" if no target support has been added in the compiler.
-fstack-limit-register=reg-fstack-limit-symbol=sym-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a
certain value, either the value of a register or the address of a
symbol. If the stack would grow beyond the value, a signal is
raised. For most targets, the signal is raised before the stack
overruns the boundary, so it is possible to catch the signal
without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000
and grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit and
-Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of
128KB. Note that this may only work with the GNU linker.
-fsplit-stack
Generate code to automatically split the stack before it overflows.
The resulting program has a discontiguous stack which can only
overflow if the program is unable to allocate any more memory.
This is most useful when running threaded programs, as it is no
longer necessary to calculate a good stack size to use for each
thread. This is currently only implemented for the i386 and x86_64
back ends running GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without
-fsplit-stack, there may not be much stack space available for the
latter code to run. If compiling all code, including library code,
with -fsplit-stack is not an option, then the linker can fix up
these calls so that the code compiled without -fsplit-stack always
has a large stack. Support for this is implemented in the gold
linker in GNU binutils release 2.21 and later.
-fleading-underscore
This option and its counterpart, -fno-leading-underscore, forcibly
change the way C symbols are represented in the object file. One
use is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate
code that is not binary compatible with code generated without that
switch. Use it to conform to a non-default application binary
interface. Not all targets provide complete support for this
switch.
-ftls-model=model
Alter the thread-local storage model to be used. The model
argument should be one of "global-dynamic", "local-dynamic",
"initial-exec" or "local-exec".
The default without -fpic is "initial-exec"; with -fpic the default
is "global-dynamic".
-fvisibility=default|internal|hidden|protected
Set the default ELF image symbol visibility to the specified
option---all symbols will be marked with this unless overridden
within the code. Using this feature can very substantially improve
linking and load times of shared object libraries, produce more
optimized code, provide near-perfect API export and prevent symbol
clashes. It is strongly recommended that you use this in any
shared objects you distribute.
Despite the nomenclature, "default" always means public; i.e.,
available to be linked against from outside the shared object.
"protected" and "internal" are pretty useless in real-world usage
so the only other commonly used option will be "hidden". The
default if -fvisibility isn't specified is "default", i.e., make
every symbol public---this causes the same behavior as previous
versions of GCC.
A good explanation of the benefits offered by ensuring ELF symbols
have the correct visibility is given by "How To Write Shared
Libraries" by Ulrich Drepper (which can be found at
<http://people.redhat.com/~drepper/>)---however a superior solution
made possible by this option to marking things hidden when the
default is public is to make the default hidden and mark things
public. This is the norm with DLL's on Windows and with
-fvisibility=hidden and "__attribute__ ((visibility("default")))"
instead of "__declspec(dllexport)" you get almost identical
semantics with identical syntax. This is a great boon to those
working with cross-platform projects.
For those adding visibility support to existing code, you may find
#pragmaGCCvisibility of use. This works by you enclosing the
declarations you wish to set visibility for with (for example)
#pragmaGCCvisibilitypush(hidden) and #pragmaGCCvisibilitypop.
Bear in mind that symbol visibility should be viewed aspartoftheAPIinterfacecontract and thus all new code should always specify
visibility when it is not the default; i.e., declarations only for
use within the local DSO should always be marked explicitly as
hidden as so to avoid PLT indirection overheads---making this
abundantly clear also aids readability and self-documentation of
the code. Note that due to ISO C++ specification requirements,
operator new and operator delete must always be of default
visibility.
Be aware that headers from outside your project, in particular
system headers and headers from any other library you use, may not
be expecting to be compiled with visibility other than the default.
You may need to explicitly say #pragmaGCCvisibilitypush(default)
before including any such headers.
extern declarations are not affected by -fvisibility, so a lot of
code can be recompiled with -fvisibility=hidden with no
modifications. However, this means that calls to extern functions
with no explicit visibility will use the PLT, so it is more
effective to use __attribute((visibility)) and/or #pragmaGCCvisibility to tell the compiler which extern declarations should be
treated as hidden.
Note that -fvisibility does affect C++ vague linkage entities. This
means that, for instance, an exception class that will be thrown
between DSOs must be explicitly marked with default visibility so
that the type_info nodes will be unified between the DSOs.
An overview of these techniques, their benefits and how to use them
is at <http://gcc.gnu.org/wiki/Visibility>.
-fstrict-volatile-bitfields
This option should be used if accesses to volatile bit-fields (or
other structure fields, although the compiler usually honors those
types anyway) should use a single access of the width of the
field's type, aligned to a natural alignment if possible. For
example, targets with memory-mapped peripheral registers might
require all such accesses to be 16 bits wide; with this flag the
user could declare all peripheral bit-fields as "unsigned short"
(assuming short is 16 bits on these targets) to force GCC to use
16-bit accesses instead of, perhaps, a more efficient 32-bit
access.
If this option is disabled, the compiler will use the most
efficient instruction. In the previous example, that might be a
32-bit load instruction, even though that will access bytes that do
not contain any portion of the bit-field, or memory-mapped
registers unrelated to the one being updated.
If the target requires strict alignment, and honoring the field
type would require violating this alignment, a warning is issued.
If the field has "packed" attribute, the access is done without
honoring the field type. If the field doesn't have "packed"
attribute, the access is done honoring the field type. In both
cases, GCC assumes that the user knows something about the target
hardware that it is unaware of.
The default value of this option is determined by the application
binary interface for the target processor.

ENVIRONMENT

This section describes several environment variables that affect how
GCC operates. Some of them work by specifying directories or prefixes
to use when searching for various kinds of files. Some are used to
specify other aspects of the compilation environment.
Note that you can also specify places to search using options such as
-B, -I and -L. These take precedence over places specified using
environment variables, which in turn take precedence over those
specified by the configuration of GCC.
LANGLC_CTYPELC_MESSAGESLC_ALL
These environment variables control the way that GCC uses
localization information which allows GCC to work with different
national conventions. GCC inspects the locale categories LC_CTYPE
and LC_MESSAGES if it has been configured to do so. These locale
categories can be set to any value supported by your installation.
A typical value is en_GB.UTF-8 for English in the United Kingdom
encoded in UTF-8.
The LC_CTYPE environment variable specifies character
classification. GCC uses it to determine the character boundaries
in a string; this is needed for some multibyte encodings that
contain quote and escape characters that would otherwise be
interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use
in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value
of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES
default to the value of the LANG environment variable. If none of
these variables are set, GCC defaults to traditional C English
behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary
files. GCC uses temporary files to hold the output of one stage of
compilation which is to be used as input to the next stage: for
example, the output of the preprocessor, which is the input to the
compiler proper.
GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing
-fcompare-debug to the compiler driver. See the documentation of
this option for more details.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the
names of the subprograms executed by the compiler. No slash is
added when this prefix is combined with the name of a subprogram,
but you can specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out an
appropriate prefix to use based on the pathname it was invoked
with.
If GCC cannot find the subprogram using the specified prefix, it
tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where
prefix is the prefix to the installed compiler. In many cases
prefix is the value of "prefix" when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are
used for linking.
In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with /usr/local/lib/gcc
(more precisely, with the value of GCC_INCLUDE_DIR), GCC tries
replacing that beginning with the specified prefix to produce an
alternate directory name. Thus, with -Bfoo/, GCC will search
foo/bar where it would normally search /usr/local/lib/bar. These
alternate directories are searched first; the standard directories
come next. If a standard directory begins with the configured
prefix then the value of prefix is replaced by GCC_EXEC_PREFIX when
looking for header files.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of
directories, much like PATH. GCC tries the directories thus
specified when searching for subprograms, if it can't find the
subprograms using GCC_EXEC_PREFIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories,
much like PATH. When configured as a native compiler, GCC tries
the directories thus specified when searching for special linker
files, if it can't find them using GCC_EXEC_PREFIX. Linking using
GCC also uses these directories when searching for ordinary
libraries for the -l option (but directories specified with -L come
first).
LANG
This variable is used to pass locale information to the compiler.
One way in which this information is used is to determine the
character set to be used when character literals, string literals
and comments are parsed in C and C++. When the compiler is
configured to allow multibyte characters, the following values for
LANG are recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the
compiler will use mblen and mbtowc as defined by the default locale
to recognize and translate multibyte characters.
Some additional environments variables affect the behavior of the
preprocessor.
CPATHC_INCLUDE_PATHCPLUS_INCLUDE_PATHOBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a
special character, much like PATH, in which to look for header
files. The special character, "PATH_SEPARATOR", is target-
dependent and determined at GCC build time. For Microsoft Windows-
based targets it is a semicolon, and for almost all other targets
it is a colon.
CPATH specifies a list of directories to be searched as if
specified with -I, but after any paths given with -I options on the
command line. This environment variable is used regardless of
which language is being preprocessed.
The remaining environment variables apply only when preprocessing
the particular language indicated. Each specifies a list of
directories to be searched as if specified with -isystem, but after
any paths given with -isystem options on the command line.
In all these variables, an empty element instructs the compiler to
search its current working directory. Empty elements can appear at
the beginning or end of a path. For instance, if the value of
CPATH is ":/special/include", that has the same effect as
-I.-I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output
dependencies for Make based on the non-system header files
processed by the compiler. System header files are ignored in the
dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which
case the Make rules are written to that file, guessing the target
name from the source file name. Or the value can have the form
filetarget, in which case the rules are written to file file using
target as the target name.
In other words, this environment variable is equivalent to
combining the options -MM and -MF, with an optional -MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above),
except that system header files are not ignored, so it implies -M
rather than -MM. However, the dependence on the main input file is
omitted.

BUGS

For instructions on reporting bugs, see <http://gcc.gnu.org/bugs.html>.

FOOTNOTES

1. On some systems, gcc-shared needs to build supplementary stub code
for constructors to work. On multi-libbed systems, gcc-shared
must select the correct support libraries to link against. Failing
to supply the correct flags may lead to subtle defects. Supplying
them in cases where they are not necessary is innocuous.

SEE ALSO

AUTHOR

See the Info entry for gcc, or
<http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors
to GCC.

COPYRIGHT

Copyright (c) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010,
2011, 2012 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "GNU General Public License" and "Funding Free
Software", the Front-Cover texts being (a) (see below), and with the
Back-Cover Texts being (b) (see below). A copy of the license is
included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
gcc-4.7.4 2014-06-12 GCC(1)